While userspace can detect discontinuity errors, it is useful to
also let Kernelspace reporting discontinuity, as it can help to
identify if the data loss happened either at Kernel or userspace side.

Update documentation accordingly.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

The "reserved" field was a way, used at V4L2 API, to add new
data to existing structs without breaking userspace. However,
there are now clever ways of doing that, without needing to add
an uneeded overhead. So, get rid of them.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

5 new ioctls were added to the DVB demux API, in order to
handle memory maped I/O. Add documentation for them.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>