percpu-rw-semaphore.txt: standardize document format

percpu-rw-semaphore.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:

This document is already adopting the standard format,
with a single exception: we're using this convention
for the document title:
	===
	foo
	===

So, adjust the title of this document to follow the
standard.

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