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>

Author: peff

Documentation/config/remote.txt

@@ -5,10 +5,16 @@ remote.pushDefault::
 
 remote.<name>.url::
 	The URL of a remote repository.  See linkgit:git-fetch[1] or
-	linkgit:git-push[1].
+	linkgit:git-push[1]. A configured remote can have multiple URLs;
+	in this case the first is used for fetching, and all are used
+	for pushing (assuming no `remote.<name>.pushurl` is defined).
 
 remote.<name>.pushurl::
 	The push URL of a remote repository.  See linkgit:git-push[1].
+	If a `pushurl` option is present in a configured remote, it
+	is used for pushing instead of `remote.<name>.url`. A configured
+	remote can have multiple push URLs; in this case a push goes to
+	all of them.
 
 remote.<name>.proxy::
 	For remotes that require curl (http, https and ftp), the URL to