document --exec for git-push

The text is just copied from git-send-pack.txt.

Signed-off-by: Uwe Kleine-K,Av(Bnig <zeisberg@informatik.uni-freiburg.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-push.txt

@@ -8,7 +8,7 @@ git-push - Update remote refs along with associated objects
 
 SYNOPSIS
 --------
-'git-push' [--all] [--tags] [-f | --force] <repository> <refspec>...
+'git-push' [--all] [--tags] [--exec=<receive-pack>] [-f | --force] <repository> <refspec>...
 
 DESCRIPTION
 -----------
@@ -67,6 +67,12 @@ the remote repository.
 	addition to refspecs explicitly listed on the command
 	line.
 
+\--exec::
+	Path to the 'git-receive-pack' program on the remote
+	end.  Sometimes useful when pushing to a remote
+	repository over ssh, and you do not have the program in
+	a directory on the default $PATH.
+
 -f, \--force::
 	Usually, the command refuses to update a remote ref that is
 	not a descendant of the local ref used to overwrite it.