isa.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 the main title standard for this document;
- replace _foo_ by **foo** for emphasis;
- mark literal blocks as such.

Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>