Documentation/HOWTO: improve some markups to make it visually better

Do a series of minor improvements at the ReST output format:

- Instead of using the quote blocks (::) for quotes, use
italics. That looks nicer on epub (and html) output, as
no scroll bar will be added. Also, it will adjust line
breaks on the text automatically.

- Add a missing reference to SubmittingPatches.rst and use
**foo** instead of _foo_.

- use bold for "The Perfect Patch" by removing a newline.

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