doc: git-push: create PUSH RULES section

doc: git-push: create PUSH RULES section

Right now the rules for when a `git push` is allowed are buried at the
bottom of the description of `<refspec>`. Put them in their own section
so that we can reference them from `--force` and give some context for
why they exist.

Having the "PUSH RULES" section also lets us be a little bit more
specific with the rule in `--force`: we can just focus on the rule
for pushing for a branch (which is likely the one that's most relevant)
and leave the details about what happens when you push to a tag or a ref
that isn't a branch to the later section.

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