memory-hotplug.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:

- use :Created: and :Updated: for the timestamps;
- comment its internal index;
- adjust titles and use proper markup;
- Whitespace fixes;
- Use cross references where needed;
- Use bulleted lists where needed;
- mark literal blocks;
- Use the ReST notation for a table.

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