media: dvb uAPI docs: Prefer use "Digital TV instead of "DVB"

The usage of the term "DVB" at the dvb API docs is confusing,
as, right now, it can refer to either the European digital TV
standard or to the subsystem.

So, prefer calling it as "Digital TV" on most places, to avoid
ambiguity.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>