[media] demux.h: move documentation overview from device-drivers.tmpl

It is better to keep the documentation overview at the header file,
as this makes easier for developers to remember to fix when needed.

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