media: Documentation: v4l: Fix async sensor subdev helper documentation

Document that the notifier of an async sub-device is, besider
unregistered, also cleaned up using v4l2_async_unregister_subdev().

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>