Update 6.0 guide

Author: zli82016

mmv1/third_party/terraform/website/docs/guides/version_6_upgrade.html.markdown

@@ -216,6 +216,15 @@ on the resource through Terraform and the default labels configured on the provi
 * The new output-only `effective_labels` field lists all of labels present on the resource
 in GCP, including the labels configured through Terraform, the system, and other clients.
 
+## Data source: `google_container_cluster`
+
+### Three label-related fields are now present
+
+All three of `resource_labels`, `effective_labels` and `terraform_labels` will now be present.
+All of these three fields include all of the labels present on the resource in GCP including
+the labels configured through Terraform, the system, and other clients, equivalent to
+`effective_labels` on the resource.
+
 ## Resource: `google_edgenetwork_network`
 
 ### Three label-related fields are now present