Documentation/stable_kernel_rules.txt: convert it to ReST markup

- use ReST markups for section headers;
- add cross-references to the options;
- mark code blocks;
- a few minor changes to make Sphinx happy.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>