Documentation/git-push: document that 'simple' is the default

Documentation/git-push: document that 'simple' is the default

The default behavior is well documented already in git-config(1), but
git-push(1) itself did not mention it at all. For users willing to learn
how "git push" works but not how to configure it, this makes the
documentation cumbersome to read.

Make the git-push(1) page self-contained by adding a short summary of
what 'push.default=simple' does, early in the page.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>