doc: git-push: add explanation of `git push origin main`

What happens if you run `git push` without any arguments is actually
extremely complex to explain, as discussed in the previous commit.

But it's very easy to explain what `git push <remote> <branch>` does, so
start the man page by explaining what that does.

The hope is that someone could just stop reading the man page here and
never learn anything else about `git push`, and that would be fine.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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