[media] DocBook/frontend.xml: Recomend the usage of the new API

The old way of setting delivery system parameters were to use
an union with specific per-system parameters. However, as newer
delivery systems required more data, the structure size weren't
enough to fit. So, recomend using the DVBS2API instead.

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