[media] DocBook: document the other structs/enums of demux.h

Document the following data types:
	struct dmx_ts_feed
	struct dmx_section_filter
	struct dmx_section_feed
	enum dmx_frontend_source
	struct dmx_frontend

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