[media] video.rst: a sensor is also considered to be a physical input

Add the line "Camera sensors are also considered to be a video input."

In practice all non-MC drivers for sensors support the input ioctls, and the
compliance test actually tests for the presence of these ioctls. So clarify
the documentation by explicitly mentioning sensors.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>