docs-rst: improve output for .. notes:: on LaTeX

The output for those notes are bad in pdf, as they're not
in a box with a different color. Also, it causes the output
to not build if the note is inside a table.

Change its implementation to avoid the above troubles.

The logic there came from:
	https://stackoverflow.com/questions/606746/how-to-customize-an-existing-latex-environment-without-interfering-with-other-en

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