media: dtv-core.rst: explain how to get DVBv5 statistics

It is not trivial to implement the logic that collects DVBv5
statistics. As we're seein lately too many implementations
that are not quite right when reviewing patchsets, add a
detailed explanation, adding a few examples about the right
thing to be done.

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