[media] DocBook/dvb: Update to include DVB-T2 additions

A few new capabilities added to frontend.h for DVB-T2. Added these
to the documentation plus some notes explaining that they are
used by the T2 delivery system.

Signed-off-by: Steve Kerrison <steve@stevekerrison.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>