Documentation/push: clarify matching refspec behavior

Documentation/push: clarify matching refspec behavior

The previous text was correct, but it was easy to miss the
fact that we are talking about "matching" refs. That is, the
text can be parsed as "we push the union of the sets
of remote and local heads" and not "we push the intersection
of the sets of remote and local heads". (The former actually
doesn't make sense if you think about it, since we don't
even _have_ some of those heads). A careful reading would
reveal the correct meaning, but it makes sense to be as
explicit as possible in documentation.

We also explicitly use and introduce the term "matching";
this is a term discussed on the list, and it seems useful
to for users to be able to refer to this behavior by name.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>