[media] DocBook: finish documenting struct dmx_demux

There are two callbacks still not documented:
	.//drivers/media/dvb-core/demux.h:422: warning: No description found for parameter 'get_pes_pids'
	.//drivers/media/dvb-core/demux.h:422: warning: No description found for parameter 'get_stc'

The purpose of first one is clear. The second one is used only
on the obsolete av7110 driver, and its purpose is not clear,
as it just returns a 64-bit word from the firmware to userspace.

Let's document get_pes_pids and mark get_stc as private, adding
a comment to not use it, while this is not documented.

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