media: userspace-api/media: finalize stateless FWHT codec docs

Document V4L2_CTRL_TYPE_FWHT_PARAMS and p_fwht_params. Also
improve the V4L2_PIX_FMT_FWHT_STATELESS description.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>