[media] DocBook: document typedef dmx_section_cb at demux.h

The dvb/kdapi.tmpl has already an extensive documentation about
this callback. Now that we've added function typedefs at kernel-doc,
add such documentation at demux.h, for it to appear at device-drivers
DocBook.

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