Document receive.denyDeleteCurrent

This option was introduced by 747ca24 (receive-pack:
receive.denyDeleteCurrent, 2009-02-08) but never documented.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1558,6 +1558,10 @@ receive.denyDeletes::
 	If set to true, git-receive-pack will deny a ref update that deletes
 	the ref. Use this to prevent such a ref deletion via a push.
 
+receive.denyDeleteCurrent::
+	If set to true, git-receive-pack will deny a ref update that
+	deletes the currently checked out branch of a non-bare repository.
+
 receive.denyCurrentBranch::
 	If set to true or "refuse", git-receive-pack will deny a ref update
 	to the currently checked out branch of a non-bare repository.