DOC: logs: the format directive was missing from the second log part

The "log" statement appears both in the global section and in listeners.
The "format" directive was only documented in the first one. Maybe we
should think about moving this definition to the log section by now.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 096a1fa..7679998 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -5066,7 +5066,7 @@
 
 
 log global
-log <address> [len <length>] <facility> [<level> [<minlevel>]]
+log <address> [len <length>] [format <format>] <facility> [<level> [<minlevel>]]
 no log
   Enable per-instance logging of events and traffic.
   May be used in sections :   defaults | frontend | listen | backend
@@ -5116,6 +5116,15 @@
                generally fine for all standard usages. Some specific cases of
                long captures or JSON-formatted 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