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>

Documentation/git-push.txt

@@ -43,6 +43,12 @@ to fast forward the remote ref that matches <dst>.  If
 the optional plus `+` is used, the remote ref is updated
 even if it does not result in a fast forward update.
 +
+Note: If no explicit refspec is found, (that is neither
+on the command line nor in any Push line of the
+corresponding remotes file---see below), then all the
+refs that exist both on the local side and on the remote
+side are updated.
++
 Some short-cut notations are also supported.
 +
 * `tag <tag>` means the same as `refs/tags/<tag>:refs/tags/<tag>`.