docs-rst: conf.py: adjust the size of .. note:: tag

While the current implementation works well when using as a
paragraph, it doesn't work properly if inside a table. As we
have quite a few such cases, fix the logic to take the column
size into account.

PS.: I took the logic there from the latest version of Sphinx.sty

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