Correct documentation for git-push --mirror

This option behaves more like:

  git push $url +refs/*:refs/*

than it does like:

  git push $url +refs/heads/*:refs/heads/* +refs/tags/*:refs/tags/*

so we should document it to be more clear about that.

Suggested-by: Marek Zawirski <marek.zawirski@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-push.txt

@@ -67,7 +67,8 @@ nor in any Push line of the corresponding remotes file---see below).
 
 --mirror::
 	Instead of naming each ref to push, specifies that all
-	refs under `$GIT_DIR/refs/heads/` and `$GIT_DIR/refs/tags/`
+	refs under `$GIT_DIR/refs/` (which includes but is not
+	limited to `refs/heads/`, `refs/remotes/`, and `refs/tags/`)
 	be mirrored to the remote repository.  Newly created local
 	refs will be pushed to the remote end, locally updated refs
 	will be force updated on the remote end, and deleted refs