[media] DocBook media: s/input stream/capture stream/

The term "input stream" is confusing since it is not clear whether this is an
input stream from the point of view of the application or from the point of
view of the hardware. So replace it with the more standard term "capture stream".

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>