circular-buffers.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:

- Mark titles with ReST notation;
- comment the contents table;
- Use :Author: tag for authorship;
- mark literal blocks as such;
- use valid numbered list markups;
- Don't capitalize titles.

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