[media] DocBook/net.xml: Synchronize Network data structure

There's no documentation at all for the DVB net API. Still, better to
remove a few warnings about the missing symbols. So, add the net data
structure inside the net.xml. Now, only the ioctl documentation is missed.

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