media: dvb/frontend.h: document the uAPI file

Most of the stuff at the Digital TV frontend header file
are documented only at the Documentation. However, a few
kernel-doc markups are there, several of them with parsing
issues.

Add the missing documentation, copying definitions from the
Documentation when it applies, fixing some bugs.

Please notice that DVBv3 stuff that were deprecated weren't
commented by purpose. Instead, they were clearly tagged as
such.

This patch prepares to move part of the documentation from
Documentation/ to kernel-doc comments.

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