media: Documentation: Improve v4l2_subdev_{en,dis}able_streams documentation

Document that callers of v4l2_subdev_{en,dis}able_streams() need to set
the mask to BIT_ULL(0).

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: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>