parport-lowlevel.txt: standardize document format

Each text file under Documentation follows a different
format. This one uses a man-page like approach.

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

- Mark titles;
- Mark literals and literal blocks;
- Adjust identation.

Still, the best would be to move its contents to kernel-docs.

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