[media] v4l2-framework.txt: document debug attribute

The debug attribute in /sys/class/video4linux/<devX>/debug was never
documented. Add this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>