media: Documentation: media: *_DEFAULT targets for subdevs

Some sensors have optical blanking areas, this is, pixels that are
painted and do not account for light, only noise.

These special pixels are very useful for calibrating the sensor, but
should not be displayed on a DEFAULT target.

Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>