usb.rst: Enrich its ReST representation

- use the proper warning and note markups;
- add references for parts of the document that will be
  cross-referenced on other USB docs;
- some minor adjustments to make it better to read in
  text mode and in html.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>