DOC: expand the docs for the provided stats.

Indicate for each statistic which types may have a value for
that statistic.

Explain some of the provided statistics a little more deeply.
(cherry picked from commit ebe62d645b45aa2210ef848fa16805a0aba7d75a)
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 73195ba..8407500 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -13075,44 +13075,76 @@
 do not insert any column before these ones in order not to break tools which
 use hard-coded column positions.
 
-  0. pxname: proxy name
-  1. svname: service name (FRONTEND for frontend, BACKEND for backend, any name
-    for server)
-  2. qcur: current queued requests
-  3. qmax: max queued requests
-  4. scur: current sessions
-  5. smax: max sessions
-  6. slim: sessions limit
-  7. stot: total sessions
-  8. bin: bytes in
-  9. bout: bytes out
- 10. dreq: denied requests
- 11. dresp: denied responses
- 12. ereq: request errors
- 13. econ: connection errors
- 14. eresp: response errors (among which srv_abrt)
- 15. wretr: retries (warning)
- 16. wredis: redispatches (warning)
- 17. status: status (UP/DOWN/NOLB/MAINT/MAINT(via)...)
- 18. weight: server weight (server), total weight (backend)
- 19. act: server is active (server), number of active servers (backend)
- 20. bck: server is backup (server), number of backup servers (backend)
- 21. chkfail: number of failed checks
- 22. chkdown: number of UP->DOWN transitions
- 23. lastchg: last status change (in seconds)
- 24. downtime: total downtime (in seconds)
- 25. qlimit: queue limit
- 26. pid: process id (0 for first instance, 1 for second, ...)
- 27. iid: unique proxy id
- 28. sid: service id (unique inside a proxy)
- 29. throttle: warm up status
- 30. lbtot: total number of times a server was selected
- 31. tracked: id of proxy/server if tracking is enabled
- 32. type (0=frontend, 1=backend, 2=server, 3=socket)
- 33. rate: number of sessions per second over last elapsed second
- 34. rate_lim: limit on new sessions per second
- 35. rate_max: max number of new sessions per second
- 36. check_status: status of last health check, one of:
+In brackets after each field name are the types which may have a value for
+that field. The types are L (Listeners), F (Frontends), B (Backends), and
+S (Servers).
+
+  0. pxname [LFBS]: proxy name
+  1. svname [LFBS]: service name (FRONTEND for frontend, BACKEND for backend,
+     any name for server/listener)
+  2. qcur [..BS]: current queued requests. For the backend this reports the
+     number queued without a server assigned.
+  3. qmax [..BS]: max value of qcur
+  4. scur [LFBS]: current sessions
+  5. smax [LFBS]: max sessions
+  6. slim [LFBS]: configured session limit
+  7. stot [LFBS]: cumulative number of connections
+  8. bin [LFBS]: bytes in
+  9. bout [LFBS]: bytes out
+ 10. dreq [LFB.]: requests denied because of security concerns.
+     - For tcp this is because of a matched tcp-request content rule.
+     - For http this is because of a matched http-request or tarpit rule.
+ 11. dresp [LFBS]: responses denied because of security concerns.
+     - For http this is because of a matched http-request rule, or
+       "option checkcache".
+ 12. ereq [LF..]: request errors. Some of the possible causes are:
+     - early termination from the client, before the request has been sent.
+     - read error from the client
+     - client timeout
+     - client closed connection
+     - various bad requests from the client.
+     - request was tarpitted.
+ 13. econ [..BS]: number of requests that encountered an error trying to
+     connect to a backend server. The backend stat is the sum of the stat
+     for all servers of that backend, plus any connection errors not
+     associated with a particular server (such as the backend having no
+     active servers).
+ 14. eresp [..BS]: response errors. srv_abrt will be counted here also.
+     Some other errors are:
+     - write error on the client socket (won't be counted for the server stat)
+     - failure applying filters to the response.
+ 15. wretr [..BS]: number of times a connection to a server was retried.
+ 16. wredis [..BS]: number of times a request was redispatched to another
+     server. The server value counts the number of times that server was
+     switched away from.
+ 17. status [LFBS]: status (UP/DOWN/NOLB/MAINT/MAINT(via)...)
+ 18. weight [..BS]: server weight (server), total weight (backend)
+ 19. act [..BS]: server is active (server), number of active servers (backend)
+ 20. bck [..BS]: server is backup (server), number of backup servers (backend)
+ 21. chkfail [...S]: number of failed checks. (Only counts checks failed when
+     the server is up.)
+ 22. chkdown [..BS]: number of UP->DOWN transitions. The backend counter counts
+     transitions to the whole backend being down, rather than the sum of the
+     counters for each server.
+ 23. lastchg [..BS]: number of seconds since the last UP<->DOWN transition
+ 24. downtime [..BS]: total downtime (in seconds). The value for the backend
+     is the downtime for the whole backend, not the sum of the server downtime.
+ 25. qlimit [...S]: configured maxqueue for the server, or nothing in the
+     value is 0 (default, meaning no limit)
+ 26. pid [LFBS]: process id (0 for first instance, 1 for second, ...)
+ 27. iid [LFBS]: unique proxy id
+ 28. sid [L..S]: server id (unique inside a proxy)
+ 29. throttle [...S]: current throttle percentage for the server, when
+     slowstart is active, or no value if not in slowstart.
+ 30. lbtot [..BS]: total number of times a server was selected, either for new
+     sessions, or when re-dispatching. The server counter is the number
+     of times that server was selected.
+ 31. tracked [...S]: id of proxy/server if tracking is enabled.
+ 32. type [LFBS]: (0=frontend, 1=backend, 2=server, 3=socket/listener)
+ 33. rate [.FBS]: number of sessions per second over last elapsed second
+ 34. rate_lim [.F..]: configured limit on new sessions per second
+ 35. rate_max [.FBS]: max number of new sessions per second
+ 36. check_status [...S]: status of last health check, one of:
         UNK     -> unknown
         INI     -> initializing
         SOCKERR -> socket error
@@ -13129,31 +13161,36 @@
         L7TOUT  -> layer 7 (HTTP/SMTP) timeout
         L7RSP   -> layer 7 invalid response - protocol error
         L7STS   -> layer 7 response error, for example HTTP 5xx
- 37. check_code: layer5-7 code, if available
- 38. check_duration: time in ms took to finish last health check
- 39. hrsp_1xx: http responses with 1xx code
- 40. hrsp_2xx: http responses with 2xx code
- 41. hrsp_3xx: http responses with 3xx code
- 42. hrsp_4xx: http responses with 4xx code
- 43. hrsp_5xx: http responses with 5xx code
- 44. hrsp_other: http responses with other codes (protocol error)
- 45. hanafail: failed health checks details
- 46. req_rate: HTTP requests per second over last elapsed second
- 47. req_rate_max: max number of HTTP requests per second observed
- 48. req_tot: total number of HTTP requests received
- 49. cli_abrt: number of data transfers aborted by the client
- 50. srv_abrt: number of data transfers aborted by the server (inc. in eresp)
- 51. comp_in: number of HTTP response bytes fed to the compressor
- 52. comp_out: number of HTTP response bytes emitted by the compressor
- 53. comp_byp: number of bytes that bypassed the HTTP compressor (CPU/BW limit)
- 54. comp_rsp: number of HTTP responses that were compressed
- 55. lastsess: number of seconds since last session assigned to server/backend
- 56. last_chk: last health check contents or textual error
- 57. last_agt: last agent check contents or textual error
- 58. qtime: the average queue time in ms over the 1024 last requests
- 59. ctime: the average connect time in ms over the 1024 last requests
- 60. rtime: the average response time in ms over the 1024 last requests (0 for TCP)
- 61. ttime: the average total session time in ms over the 1024 last requests
+ 37. check_code [...S]: layer5-7 code, if available
+ 38. check_duration [...S]: time in ms took to finish last health check
+ 39. hrsp_1xx [.FBS]: http responses with 1xx code
+ 40. hrsp_2xx [.FBS]: http responses with 2xx code
+ 41. hrsp_3xx [.FBS]: http responses with 3xx code
+ 42. hrsp_4xx [.FBS]: http responses with 4xx code
+ 43. hrsp_5xx [.FBS]: http responses with 5xx code
+ 44. hrsp_other [.FBS]: http responses with other codes (protocol error)
+ 45. hanafail [...S]: failed health checks details
+ 46. req_rate [.F..]: HTTP requests per second over last elapsed second
+ 47. req_rate_max [.F..]: max number of HTTP requests per second observed
+ 48. req_tot [.F..]: total number of HTTP requests received
+ 49. cli_abrt [..BS]: number of data transfers aborted by the client
+ 50. srv_abrt [..BS]: number of data transfers aborted by the server
+     (inc. in eresp)
+ 51. comp_in [.FB.]: number of HTTP response bytes fed to the compressor
+ 52. comp_out [.FB.]: number of HTTP response bytes emitted by the compressor
+ 53. comp_byp [.FB.]: number of bytes that bypassed the HTTP compressor
+     (CPU/BW limit)
+ 54. comp_rsp [.FB.]: number of HTTP responses that were compressed
+ 55. lastsess [..BS]: number of seconds since last session assigned to
+     server/backend
+ 56. last_chk [...S]: last health check contents or textual error
+ 57. last_agt [...S]: last agent check contents or textual error
+ 58. qtime [..BS]: the average queue time in ms over the 1024 last requests
+ 59. ctime [..BS]: the average connect time in ms over the 1024 last requests
+ 60. rtime [..BS]: the average response time in ms over the 1024 last requests
+     (0 for TCP)
+ 61. ttime [..BS]: the average total session time in ms over the 1024 last
+     requests
 
 
 9.2. Unix Socket commands