[media] DocBook: Improve xref check for undocumented ioctls

There are several badly documented undocumented ioctls.
Currently, it just generates an empty link. Instead of doing that,
only add references to the ones that exists, and add a warning
for all references that weren't found.

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