siphash.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;
- Mark literal blocks;
- Use :Author: for authorship;
- Don't sumerate chapters;
- Adjust identation.

NOTE:

This file has actually two documents inside it, the first
one describing siphash, the second one describing halfsiphash.

It is likely a good idea to split them when it gets moved to
security/ (which is where it probably belongs).

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