Add usage of regiona PD in zonal clusters to user guide

Author: TeweiLuo

docs/kubernetes/user-guides/basic.md

@@ -46,18 +46,20 @@ web-server                1/1       Running   0          1m
 **Note:** Regional cluster support only available in beta starting with
 Kubernetes 1.14.
 
-This example provisions a regional PD in regional clusters.
+This example provisions a regional PD in either zonal or regional clusters.
 
-1. Create example Regional Storage Class. Choose between:
+1. Create example Regional Storage Class. For zonal clusters, `allowedTopologies` must be specified with two zones where one of the zones must be the cluster's zone. For regional clusters this configuration can also be used to make sure that regional PD is provisioned in those two zones.
 
-    * Unrestricted zones
+    * Restricted zones
       ```
-      $ kubectl apply -f ./examples/kubernetes/demo-regional-sc.yaml
+      $ kubectl apply -f ./examples/kubernetes/demo-regional-restricted-sc.yaml
       ```
 
-    * Restricted zones
+    For regional clusters it is also possible to not specify `allowedTopologies`, in which case two zones will be picked from the available zones in the cluster's region.
+
+    * Unrestricted zones
       ```
-      $ kubectl apply -f ./examples/kubernetes/demo-regional-restricted-sc.yaml
+      $ kubectl apply -f ./examples/kubernetes/demo-regional-sc.yaml
       ```
 
 2. Create example PVC and Pod