pwm: Standardize document format

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

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

- mark document title;
- mark literal blocks;
- better format the parameters.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>