media: v4l2-dev.h: add kernel-doc to two macros

There are two macros at v4l2-dev.h that aren't documented.

Document them, for completeness.

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