nommu-mmap.txt: standardize document format

nommu-mmap.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

- Adjust identation for main title;
- fix level for chapter titles;
- use ".. important::" tag for an important note;
- use the right notation for paragraph auto-numbering "(#)";
- Fix footnotes syntax;
- fix one literal var to use the right ReST tag.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>