irqflags-tracing.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:

There isn't much to be done here: just mark the document
title as such and add a :Author:.

While here, use upper case at the beginning of a few paragraphs
that start with lower case.

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