[media] media.h: let be clear that tuners need to use connectors

The V4L2 core won't be adding connectors to the tuners and other
entities that need them. Let it be clear.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>