[media] DocBook: Add documentation about the demux API

There are several stuff at media's kdapi.xml that don't
belong there, as it documents the Kernel internal ABI, and
not the userspace API.

Add the documentation here. The hole kdapi.xml will be
removed on a latter patch, after we finish documenting
what's there at the proper places.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>