media: v4l2-subdev.rst: Update doc regarding subdev descriptors

Update the doc to describe the new method of adding subdevice
descriptors to async notifiers.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>