Document receive.advertiseatomic

This was missing in 1b70fe5 (2015-01-07, receive-pack.c: negotiate
atomic push support) as I squashed the option in very late in the patch
series.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2072,6 +2072,11 @@ rebase.autostash::
 	successful rebase might result in non-trivial conflicts.
 	Defaults to false.
 
+receive.advertiseatomic::
+	By default, git-receive-pack will advertise the atomic push
+	capability to its clients. If you don't want to this capability
+	to be advertised, set this variable to false.
+
 receive.autogc::
 	By default, git-receive-pack will run "git-gc --auto" after
 	receiving data from git-push and updating refs.  You can stop