V4L/DVB (4985): Update cx2341x documentation.

Removed a few unimplemented commands. Added a note for a few fields that are
not implemented in the firmware, and clarified several issues around reverse
playback.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>