media: docs: update avermedia.rst contents

While this is old, now that we moved the intro part of it,
its contents seem to be valid, if we mention that we're
talking only about the BT878 support.

Update the document title accordingly and remove the obsolete
note from it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>