Document that GKE master_auth key is an array

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

website/docs/r/container_cluster.html.markdown

@@ -202,19 +202,40 @@ exported:
 * `instance_group_urls` - List of instance group URLs which have been assigned
     to the cluster
 
-* `master_auth.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
-    to authenticate to the cluster endpoint
-
-* `master_auth.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
     be different than the `min_master_version` set in the config if the master
     has been updated by GKE.
 
+* `master_auth` is an array of access credentials where the first key (`0`) is always
+  populated.
+
+  The following example adds the `client_certificate`, `client_key` and
+  `cluster_ca_certificate` values to the output:
+
+  ```
+  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}"
+  }
+  ```  
+
+  * `master_auth.<key>.client_certificate` - Base64 encoded public certificate
+      used by clients to authenticate to the cluster endpoint.
+
+  * `master_auth.<key>.client_key` - Base64 encoded private key used by clients
+      to authenticate to the cluster endpoint.
+
+  * `master_auth.<key>.cluster_ca_certificate` - Base64 encoded public certificate
+      that is the root of trust for the cluster.     
+
+
 <a id="timeouts"></a>
 ## Timeouts
 
@@ -231,4 +252,4 @@ Container clusters can be imported using the `zone`, and `name`, e.g.
 
 ```
 $ terraform import google_container_cluster.mycluster us-east1-a/my-cluster
-```
+```