[media] DocBook/frontend.xml: add references for some missing info

The frontend.h.xml now references to the main document. However,
several references are missed.

Links the trivial ones with the corresponding API descriptions.

While here, updates the main API to reflect the API improvements.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
2 files changed