Document that GKE master_auth key is an array (hashicorp#617)

* Document that GKE master_auth key is an array

This resolves hashicorp/terraform#16417 and is a follow up to hashicorp/terraform#14073

* Update container_cluster.html.markdown

* Update container_cluster.html.markdown

* Update container_cluster.html.markdown

Author: arekkas

website/docs/r/container_cluster.html.markdown

@@ -49,6 +49,19 @@ resource "google_container_cluster" "primary" {
     tags = ["foo", "bar"]
   }
 }
+
+# The following outputs allow authentication and connectivity to the Google Container Cluster.
+output "client_certificate" {
+  value = "${google_container_cluster.primary.master_auth.0.client_certificate}"
+}
+
+output "client_key" {
+  value = "${google_container_cluster.primary.master_auth.0.client_key}"
+}
+
+output "cluster_ca_certificate" {
+  value = "${google_container_cluster.primary.master_auth.0.cluster_ca_certificate}"
+}
 ```
 
 ## Argument Reference
@@ -214,13 +227,13 @@ exported:
 * `instance_group_urls` - List of instance group URLs which have been assigned
     to the cluster.
 
-* `master_auth.client_certificate` - Base64 encoded public certificate
+* `master_auth.0.client_certificate` - Base64 encoded public certificate
     used by clients to authenticate to the cluster endpoint.
 
-* `master_auth.client_key` - Base64 encoded private key used by clients
+* `master_auth.0.client_key` - Base64 encoded private key used by clients
     to authenticate to the cluster endpoint.
 
-* `master_auth.cluster_ca_certificate` - Base64 encoded public certificate
+* `master_auth.0.cluster_ca_certificate` - Base64 encoded public certificate
     that is the root of trust for the cluster.
 
 * `master_version` - The current version of the master in the cluster. This may
@@ -243,4 +256,4 @@ Container clusters can be imported using the `zone`, and `name`, e.g.
 
 ```
 $ terraform import google_container_cluster.mycluster us-east1-a/my-cluster
-```
+```