media: dvbproperty.rst: improve notes about legacy frontend calls

The description of the DVBv5 API was written a long time ago,
where the API was still new, and there were not apps using it.

Now that the API is stable and used by new applications, clarify
that DVBv3 calls should not be used and why.

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