doc: git-push: clarify "what to push"

From user feedback: 6 users says they found the "what to push"
paragraphs confusing, for many different reasons, including:

* what does "..." in <refspec>... mean?
* "consult XXX configuration" is hard to parse
* it refers to the `git-config` man page even though the config
information for `git push` is included in this man page under
CONFIGURATION
* the default ("push to a branch with the same name") is what they use
99% of the time, they would have expected it to appear earlier instead
of at the very end
* not understanding what the term "upstream" means in Git
("are branches tracked by some system besides their names?"")

Also, the current explanation of `push.default=simple` ("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.") is not accurate:
`push.default=simple` does not always require you to set a corresponding
upstream branch.

Address all of these by

* using a numbered "in order of precedence" list
* giving a more accurate explanation of how `push.default=simple` works
* giving a little bit of context around "upstream branch": it's
something that you may have to set explicitly
* referring to the new UPSTREAM BRANCHES section

The default behaviour is still discussed pretty late but it should be
easier to skim now to get to the relevant information.

In "`git push` may fail if...", I'm intentionally being vague about
what exactly `git push` does, because (as discussed on the mailing list)
the behaviour of `push.default=simple` is very confusing, perhaps broken,
and certainly not worth trying to explain in an introductory context.
`push.default.simple` sometimes requires you to set an upstream and
sometimes doesn't and the exact conditions under which it does/doesn't
are hard to describe.

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