[proposal] Warn google_compute_disk_resource_policy_attachment for google_compute_region_disk

[wata727]

See https://www.terraform.io/docs/providers/google/r/compute_disk_resource_policy_attachment.html

Note: This resource does not support regional disks (google_compute_region_disk). For regional disks, please refer to google_compute_region_disk_resource_policy_attachment