media: Clarify v4l2-async subdevice addition API

Now that most users of v4l2_async_notifier_add_subdev have been converted,
let's fix the documentation so it's more clear how the v4l2-async API
should be used.

Document functions that drivers should use, and their purpose.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>