docs: update http.<url>.* options documentation

Overhaul the text of the http.<url>.* options documentation
providing a hopefully easier to understand itemized list of matching
behavior as suggested by and including text from Jeff King.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>