kernel-per-CPU-kthreads.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 title markups;
- use "-" for bulletted lists;
- Split Name/Purpose on two lines, in order to make visually
  easier to read (in text format), and to bold the title
  (on ReST output)
- Add blank lines to split bulleted lists;
- use sub-titles for the several kthread softirq types;
- mark one literal var with asterisk as such, in order to
  avoid an error warning on Sphinx.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>