docs/resource/aws_appautoscaling_policy: Additional customized_metric_specification documentation (#12696)
docs/resource/aws_appautoscaling_policy: Additional customized_metric_specification documentation (#12696)

* docs/resource/aws_appautoscaling_policy: Additional customized_metric_specification documentation

Reference: https://github.com/terraform-providers/terraform-provider-aws/issues/11627
Reference: https://docs.aws.amazon.com/autoscaling/application/userguide/application-auto-scaling-target-tracking.html

* docs/resource/aws_appautoscaling_policy: Adjust headers to remove code spans and no deeper than H3

Reference: https://github.com/terraform-providers/terraform-provider-aws/pull/12696#pullrequestreview-402129873
terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws