media: subdev: Improve s_stream documentation

Now that enable/disable_streams operations are available for
single-stream subdevices too, there's no reason to use the old s_stream
operation on new drivers. Extend the documentation reflecting this.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Umang <Jain&lt;umang.jain@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>