provider/aws: Add default_cooldown to the autoscaling_group docs (#11957)

Fixes: #11947

website/source/docs/providers/aws/r/autoscaling_group.html.markdown

@@ -70,6 +70,7 @@ The following arguments are supported:
     (See also [Waiting for Capacity](#waiting-for-capacity) below.)
 * `availability_zones` - (Optional) A list of AZs to launch resources in.
    Required only if you do not specify any `vpc_zone_identifier`
+* `default_cooldown` - (Optional) The amount of time, in seconds, after a scaling activity completes before another scaling activity can start.
 * `launch_configuration` - (Required) The name of the launch configuration to use.
 * `initial_lifecycle_hook` - (Optional) One or more
   [Lifecycle Hooks](http://docs.aws.amazon.com/autoscaling/latest/userguide/lifecycle-hooks.html)