[media] demux.h: Some documentation fixups for the header

The DocBook description of this header has two issues:
- It calls the Kernel ABI as API, instead of kABI;
- It mentions that the DVB frontend kABI is not described
  within the document. As this will actually generate a
  single DocBook, this is actually not true, now that
  the documentation for the frontend was added.

So, fix both issues.

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