config: document remote.*.url/pushurl interaction

The documentation for these keys gives a very terse definition and
points you to the fetch/push manpages. But from reading those pages it
was not at all obvious to me that:

- these are keys that can be defined multiple times with meaningful
behavior (especially remote.*.url)

- the way that pushurl overrides url (the git-push page does mention
that "pushurl defaults to url", but it is not immediately clear what
a multi-valued url would do in that situation).

Let's try to summarize the current behavior.

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