http-backend: reword some documentation

http-backend: reword some documentation

Clarify some of the git-http-backend documentation, particularly:

* In the Description, state that smart/dumb HTTP fetch and smart HTTP
  push are supported, state that authenticated clients allow push, and
  remove the note that this is only suited for read-only updates.

* At the start of Examples, state explicitly what URL is mapping to what
  location on disk.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>