DOC: mention support for the RFC5424 syslog message format

Document the new log "format" argument and the "log-format-sd" statement.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index dc9e5d9..0381385 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -675,7 +675,7 @@
   Similar to "gid" but uses the GID of group name <group name> from /etc/group.
   See also "gid" and "user".
 
-log <address> [len <length>] <facility> [max level [min level]]
+log <address> [len <length>] [format <format>] <facility> [max level [min level]]
   Adds a global syslog server. Up to two global servers can be defined. They
   will receive logs for startups and exits, as well as all logs from proxies
   configured with "log global".
@@ -710,6 +710,15 @@
            standard usages. Some specific cases of long captures or
            JSON-formated logs may require larger values.
 
+  <format> is the log format used when generating syslog messages. It may be
+           one of the following :
+
+    rfc3164   The RFC3164 syslog message format. This is the default.
+              (https://tools.ietf.org/html/rfc3164)
+
+    rfc5424   The RFC5424 syslog message format.
+              (https://tools.ietf.org/html/rfc5424)
+
   <facility> must be one of the 24 standard syslog facilities :
 
           kern   user   mail   daemon auth   syslog lpr    news
@@ -1652,6 +1661,7 @@
 load-server-state-from-file               X          -         X         X
 log                                  (*)  X          X         X         X
 log-format                                X          X         X         -
+log-format-sd                             X          X         X         -
 log-tag                                   X          X         X         X
 max-keep-alive-queue                      X          -         X         X
 maxconn                                   X          X         X         -
@@ -4350,6 +4360,27 @@
   the same log format. Please see section 8.2.4 which covers the log format
   string in depth.
 
+log-format-sd <string>
+  Specifies the RFC5424 structured-data log format string
+  May be used in sections:    defaults | frontend | listen | backend
+                                 yes   |    yes   |   yes  |    no
+
+  This directive specifies the RFC5424 structured-data log format string that
+  will be used for all logs resulting from traffic passing through the frontend
+  using this line. If the directive is used in a defaults section, all
+  subsequent frontends will use the same log format. Please see section 8.2.4
+  which covers the log format string in depth.
+
+  See https://tools.ietf.org/html/rfc5424#section-6.3 for more information
+  about the RFC5424 structured-data part.
+
+  Note : This log format string will be used only for loggers that have set
+         log format to "rfc5424".
+
+  Example :
+    log-format-sd [exampleSDID@1234\ bytes=\"%B\"\ status=\"%ST\"]
+
+
 log-tag <string>
   Specifies the log tag to use for all outgoing logs
   May be used in sections:    defaults | frontend | listen | backend