DOC: add missing entry for log-format and clarify the text

There was no "log-format" entry in the keyword index! This should be
backported to 1.5.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index ee3b834..1b4be4d 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1352,6 +1352,7 @@
 id                                        -          X         X         X
 ignore-persist                            -          X         X         X
 log                                  (*)  X          X         X         X
+log-format                                X          X         X         -
 max-keep-alive-queue                      X          -         X         X
 maxconn                                   X          X         X         -
 mode                                      X          X         X         X
@@ -3547,9 +3548,15 @@
 
 
 log-format <string>
-   Allows you to custom a log line.
+  Specifies the log format string to use for traffic logs
+  May be used in sections:    defaults | frontend | listen | backend
+                                 yes   |    yes   |   yes  |    no
 
-   See also : Custom Log Format (8.2.4)
+  This directive specifies the 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.
 
 
 max-keep-alive-queue <value>