Documentation/CodingStyle: replace underline markups

Sphinx doesn't accept underline markups by purpose.
While there are ways to support underline via CSS, this won't
be portable with non-html outputs.

As we want CodingStyle to do emphasis, replace _foo_ by **foo**,
using bold emphasis.

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