[media] DocBook/frontend.xml: Better document fe_type_t

The fe_type_t documentation was incomplete and not linked to the
dvb/frontend.h.xml. Properly document it.

Also, drop a note that newer formats are only supported via
FE_GET_PROPERTY/FE_GET_SET_PROPERTY ioctls.

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