Usually, CA messages are sent/received via reading/writing at
the CA device node. However, two drivers (dst_ca and firedtv-ci)
also implement it via ioctls.

Apparently, on both cases, the net result is the same.

Anyway, let's document it.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>