media: Documentation: v4l: Document v4l2_async_nf_cleanup

Document v4l2_async_nf_cleanup() which must be called before releasing an
unregistered notifier's memory. Also remove the sentence regarding
v4l2_async_nf_init() arguments --- those are documented in kerneldoc which
is referred here.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com> # Renesas RZ/G2L SMARC
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>