media: Documentation: Additional streams generally don't harm capture

Having extra streams on the source end of the link that cannot be captured
by the sink sub-device generally are not an issue, at least not on CSI-2
bus. Still document that there may be hardware-specific limitations. For
example on parallel bus this might not work on all cases.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>