[media] media: Update documentation for media_entity_notify

Update documentation for media_entity_notify to clearly state the usage
restrictions. This handler is intended for creating links between exiting
entities and should not used to create and register entities.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>