help: clarify behavior of server.concurrent-push-mode

So it doesn't seemingly say that old clients cannot talk to server
configured with concurrent-push-mode=check-related. They can, they
just don't get the benefit.

Differential Revision: https://phab.mercurial-scm.org/D8201

mercurial/helptext/config.txt

@@ -2010,8 +2010,8 b' Controls generic server settings.'
     - 'check-related': push is only aborted if it affects head that got also
       affected while the push was preparing.
 
-    This requires compatible client (version 4.3 and later). Old client will
-    use 'strict'.
+    'check-related' only takes effect for compatible clients (version
+    4.3 and later). Older clients will use 'strict'.
 
 ``validate``
     Whether to validate the completeness of pushed changesets by