add documentation to V4L2_CID_MPEG_VIDEO_FWHT_PARAMS
control and its related 'v4l2_ctrl_fwht_params' struct

[mchehab+samsung@kernel.org: remove extra blank lines]
Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>