[media] DocBook: document DVB DMX_[ADD|REMOVE]_PID

Those ioctls were added back in 2009, at changeset 1cb662a3


but were never documented. Fortunately, the original commit is
good enough to serve as the basis for documenting it. Also, the
support for it is done by dmxdev implementation.

So, add a proper documentation for it, based on the description
of the original changeset.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>