[media] dvb: frontend.h: improve dvb_frontent_parameters comment

The comment for struct dvb_frontend_parameters is weird, as it
mixes delivery system name (ATSC) with modulation names
(QPSK, QAM, OFDM).

Use delivery system names there on the frequency comment, as this
is clearer, specially after 2GEN delivery systems.

While here, add comments at the union, to make live easier for ones
that may try to understand the convention used by the legacy API.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>