media: v4l2-async: better describe match union at async match struct

Now that kernel-doc handles nested unions, better document the
match union at struct v4l2_async_subdev.

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