The tuner ops g_frequency, g_tuner and s_tuner require that the tuner type
field is filled in. Document this.

The tuner-core doc is based on a patch from Mauro Carvalho Chehab <mchehab@redhat.com>.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>