[media] DVB: improve documentation for satellite delivery systems

- Move voltage and tone to DVB-S.
- Add turbo code.
- In DVB-S2 and turbo code sections, refer to DVB-S, as both
  are extensions to DVB-S.
- Add modulation to DVB-S2.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>