media: dvb: add continuity error indicators for memory mapped buffers

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: Mauro Carvalho Chehab <mchehab@s-opensource.com>