[media] DocBook: add a proper description for dvb_frontend_info.fe_type

The fe_type is deprecated at the DVB API. However, it may still
be used by legacy DVBv3 applications. While this works with old
devices, modern devices may support more than one delivery
system.

Add an explanation about that and a point to what should be
used, instead, in order for legacy apps to support newer hardware.

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