DOC: config: Fix pgsql-check documentation to make user param mandatory

The username is required in the Start-up message. Thus, since the 2.2, when
this health-check was refactored, the user parameter is mandatory. On prior
versions, when no username is provided, no pgsql check is performed but only
a basic tcpcheck.

This patch should be backported as far as 2.2.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index db86f17..f16c779 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -9717,7 +9717,7 @@
   See also : "option redispatch", "retries", "force-persist"
 
 
-option pgsql-check [ user <username> ]
+option pgsql-check user <username>
   Use PostgreSQL health checks for server testing
   May be used in sections :   defaults | frontend | listen | backend
                                  yes   |    no    |   yes  |   yes