[media] DocBook media: update bytesused field description

For output buffers the application has to set the bytesused field.
In reality applications often do not set this since drivers that
deal with fix image sizes just override it anyway.

The vb2 framework will replace this field with the length field if
bytesused was set to 0 by the application, which is what happens
in practice. Document this behavior.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>