media: docs: bt8xx.rst: update document info

This document is very outdated, and doesn't match the
upstream current way.

Update it, making some parts a little bit more generic.
While the main focus of this document is digital TV
cards, its content also may also help someone with just
analog TV support.

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