media: Documentation: Document S_ROUTING behaviour

Document S_ROUTING behaviour for different devices.

Generally in devices that produce streams the streams are static and some
can be enabled and disabled, whereas in devices that just transport them
or write them to memory, more configurability is allowed. Document this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Julien Massot <julien.massot@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>