Add urls.txt to git-clone man page

Add urls.txt to git-clone man page

Since git-clone is one of the many commands taking
URLs to remote repositories as an argument, it should include
the URL-types list from urls.txt.

Split up urls.txt into urls.txt and urls-remotes.txt.  The latter
should be used by anything besides git-clone where a discussion of
using .git/config and .git/remotes/ to name URLs just doesn't make
as much sense.

Signed-off-by: Andrew Ruder <andy@aeruder.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>