Add docs about regional and zonal versions differing in GKE (hashicor…

…p#128)

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/cc @rileykarson

website/docs/d/google_container_engine_versions.html.markdown

@@ -10,6 +10,12 @@ description: |-
 
 Provides access to available Google Container Engine versions in a zone or region for a given project.
 
+-> If you are using the `google_container_engine_versions` datasource with a regional cluster, ensure that you have provided a `region`
+to the datasource. A `region` can have a different set of supported versions than its corresponding `zone`s, and not all `zone`s in a 
+`region` are guaranteed to support the same version.
+
+## Example Usage
+
 ```hcl
 data "google_container_engine_versions" "central1b" {
   zone = "us-central1-b"

website/docs/r/container_cluster.html.markdown

@@ -142,6 +142,10 @@ output "cluster_ca_certificate" {
     If unset, the cluster's version will be set by GKE to the version of the most recent
     official release (which is not necessarily the latest version).
 
+-> If you are using the `google_container_engine_versions` datasource with a regional cluster, ensure that you have provided a `region`
+to the datasource. A `region` can have a different set of supported versions than its corresponding `zone`s, and not all `zone`s in a 
+`region` are guaranteed to support the same version.
+
 * `monitoring_service` - (Optional) The monitoring service that the cluster
     should write metrics to.
     Automatically send metrics from pods in the cluster to the Google Cloud Monitoring API.