docs-rst: conf.py: only setup notice box colors if Sphinx < 1.6

Sphinx 1.5 added a new way to change backward colors for note
boxes, but kept backward compatibility with 1.4. On Sphinx 1.6,
the old way stopped working, in favor of a new less hackish
way.

Unfortunately, this is currently too buggy to be used, and
the old way doesn't work anymore. So, we have no option but
to stick with boring notice boxes.

One example of such bug is the notice that it is inside
struct v4l2_plane, at the "bytesused" field.

At least, add a notice about how to use, as maybe some day
the bug will vanish.

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