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>