media: dmx.h: add kernel-doc markups and use it at Documentation/

The demux documentation is pretty poor nowadays: most of the
structs and enums aren't documented at all.

Add proper kernel-doc markups for them and use it.

Now, the demux API data structures are fully documented :-)

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>