media: Documentation: Update {enable,disable}_streams documentation

Document the expected {enable,disable}_streams callback behaviour for
drivers that are stream-unaware i.e. don't specify the
V4L2_SUBDEV_CAP_STREAMS sub-device capability flag. In this specific case,
the mask argument can be ignored.

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>