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>

Documentation/git-push.txt

@@ -36,6 +36,13 @@ the default `<refspec>` by consulting `remote.*.push` configuration,
 and if it is not found, honors `push.default` configuration to decide
 what to push (See linkgit:git-config[1] for the meaning of `push.default`).
 
+When neither the command-line nor the configuration specify what to
+push, the default behavior is used, which corresponds to the `simple`
+value for `push.default`: the current branch is pushed to the
+corresponding upstream branch, but as a safety measure, the push is
+aborted if the upstream branch does not have the same name as the
+local one.
+
 
 OPTIONS[[OPTIONS]]
 ------------------