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>