[media] DocBook: cleaup the notes about DTV properties

The notes there are somewhat confusing and assumes that the
reader would have read the DVBv3 way. This is not true anymore,
as the DVBv3 is now on a separate section that is marked as
deprecated.

So, cleanup the notes.

While here, add a note about using libdvbv5, instead of using
the DVBv5 API directly.

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