IPMI.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:

- fix document type;
- add missing markups for subitems;
- mark literal blocks;
- add whitespaces and blank lines where needed;
- use bulleted list markups where neded.

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