docs/resource/aws_appautoscaling_policy: Link to AWS documentation fo…

…r policy_type argument documentation

Reference: #3520
Reference: https://docs.aws.amazon.com/autoscaling/application/userguide/application-auto-scaling-target-tracking.html
Reference: https://docs.aws.amazon.com/autoscaling/application/userguide/application-auto-scaling-step-scaling-policies.html

Switches to AWS documentation links instead of continually being out of date.

website/docs/r/appautoscaling_policy.html.markdown

@@ -123,7 +123,7 @@ resource "aws_appautoscaling_policy" "replicas" {
 The following arguments are supported:
 
 * `name` - (Required) The name of the policy.
-* `policy_type` - (Optional) For DynamoDB, only `TargetTrackingScaling` is supported. For Amazon ECS, Spot Fleet, and Amazon RDS, both `StepScaling` and `TargetTrackingScaling` are supported. For any other service, only `StepScaling` is supported. Defaults to `StepScaling`.
+* `policy_type` - (Optional) The policy type. Valid values are `StepScaling` and `TargetTrackingScaling`. Defaults to `StepScaling`. Certain services only support only one policy type. For more information see the [Target Tracking Scaling Policies](https://docs.aws.amazon.com/autoscaling/application/userguide/application-auto-scaling-target-tracking.html) and [Step Scaling Policies](https://docs.aws.amazon.com/autoscaling/application/userguide/application-auto-scaling-step-scaling-policies.html) documentation.
 * `resource_id` - (Required) The resource type and unique identifier string for the resource associated with the scaling policy. Documentation can be found in the `ResourceId` parameter at: [AWS Application Auto Scaling API Reference](http://docs.aws.amazon.com/ApplicationAutoScaling/latest/APIReference/API_RegisterScalableTarget.html#API_RegisterScalableTarget_RequestParameters)
 * `scalable_dimension` - (Required) The scalable dimension of the scalable target. Documentation can be found in the `ScalableDimension` parameter at: [AWS Application Auto Scaling API Reference](http://docs.aws.amazon.com/ApplicationAutoScaling/latest/APIReference/API_RegisterScalableTarget.html#API_RegisterScalableTarget_RequestParameters)
 * `service_namespace` - (Required) The AWS service namespace of the scalable target. Documentation can be found in the `ServiceNamespace` parameter at: [AWS Application Auto Scaling API Reference](http://docs.aws.amazon.com/ApplicationAutoScaling/latest/APIReference/API_RegisterScalableTarget.html#API_RegisterScalableTarget_RequestParameters)