Documentation: mention `push.default` in git-push.txt
Documentation: mention `push.default` in git-push.txt

It already is listed in the "git config" documentation, but people
interested in pushing would first look at "git push" documentation.

Noticed-by: David Glasser
Signed-off-by: Ramkumar Ramachandra <5c355d1d38066ed0c5de420d67c1b5ed0687c4bd@gmail.com>
Acked-by: Matthieu Moy <d4d2cc2a6ec7b2aa587d4887cc5ed200f9b685a1@grenoble-inp.fr>
Fixed-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git