V4L/DVB (13164): DocBook/dvb: Starts documenting DVBS2API ioctls

DVBS2API introduced two new ioctls that allows changing/getting details about newer
video standards like DVB-S2 and ISDB-T. There are lots of thing to do in this chapter,
but, for now, just add a session there, in order to allow make to not complain about
undefined symbol.

With this patch, all ioctls at the frontend.h API were somehow documented.

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