[media] DocBook: Move struct dmx_demux kABI doc to demux.h

[media] DocBook: Move struct dmx_demux kABI doc to demux.h

The DocBook/media/dvb/kdapi.xml contains the description of
the kABI for DVB. The problem is that, by being maintained
on a separate file and not being updated for years, it got
outdated. So, for example, some callback parameters were
changed, but the DocBook were still using the old stuff.

As a first step to fix it, let's move the documentation of
struct dmx_demux into demux.h and fix the parameters used
there.

For now, don't document any other field nor touch the
descriptions that got moved, letting this job to other
patches. That makes easier to review the patch.

PS.: Please notice that an additional patch will be needed
in order to fix the return values (some uses non-existent
return codes) and to the functions and callbacks mentioned at
the descriptions.

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