writing_musb_glue_layer.rst: Enrich its ReST representation

This file is actually quite complex, and required several
manual handwork:

- add a title for the document;
- use the right tags for monospaced fonts;
- use c references where needed;
- adjust cross-reference to writing_usb_driver.rst
- hightlight cross-referenced lines.

With regards to C code snippet line highlights, the better would be
to use :linenos: for the C code snippets that are referenced by
the line number. However, at least with Sphinx 1.4.9, enabling
it cause the line number to be misaligned with the code,
making it even more confusing. So, instead, let's use
:emphasize-lines: tag to mark the lines that are referenced
at the text.
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>