[media] doc-rst: mention the memory type to be set for all streaming I/O

Changeset 8c9f46095176 ("[media] DocBook: mention the memory type to
be set for all streaming I/O") updated the media DocBook to mention
the need of filling the memory types. We need to keep the ReST
doc updated to such change.

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