media: v4l2-subdev: document remaining undocumented functions

There are several undocumented v4l2-subdev functions that are
part of kAPI. Document them.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>