media: subdev: Improve v4l2_subdev_get_frame_desc_passthrough() kdoc

media: subdev: Improve v4l2_subdev_get_frame_desc_passthrough() kdoc

Improve the v4l2_subdev_get_frame_desc_passthrough() kernel doc:

- Fix 'v4l2_get_frame_desc' operation to 'get_frame_desc' operation
- Rewrite the body text to be more understandable and specific, and
  specifically mention the frame desc type handling.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>