[media] DocBook: Multistream support

Document the multistream support at the DVBAPI, describing
what delivery systems currently support it (DVB-T2, DVB-S2,
ISDB-S).

DVBAPI version increased to 5.8.

[mchehab@redhat.com: Made some corrections - mostly language]
Signed-off-by: Evgeny Plehov <EvgenyPlehov@ukr.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>