media: docs: update bttv.rst information

This document is... old. The bttv driver was one of the first
drivers at the Kernel. So, the document became a little obsoleted.

Update it to reflect some changes that happened along the time
affecting this driver and the subsystem.

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