git-push: Update documentation to describe the no-refspec behavior.

git-push: Update documentation to describe the no-refspec behavior.

It turns out that the git-push documentation didn't describe what it
would do when not given a refspec, (not on the command line, nor in a
remotes file). This is fairly important for the user who is trying to
understand operations such as:

	git clone git://something/some/where
	# hack, hack, hack
	git push origin

I tracked the mystery behavior down to git-send-pack and lifted the
relevant portion of its documentation up to git-push, (namely that all
refs existing both locally and remotely are updated).

Signed-off-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>