Documentation/stable_api_nonsense.txt: convert it to ReST markup

Add markups for it to be properly parsed by Sphinx.

As people browsing this document may not notice that the source
file title is "stable_api_nonsense", I opted to use bold to
the rationale for this document. I also found it better to
add a note when it says that the nonsense applies only to the
kABI/kAPI, and not to uAPI.

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