V4L/DVB (13162): Docbook/dvb: add missing specs for FE_DISHNETWORK_SEND_LEGACY_CMD ioctl

While this ioctl should not be used, and is a good canditate for
Documentation/feature-removal-schedule.txt, while it is on kernel,
it needs to be documented.

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