Update statefulset concepts doc to use apps/v1beta2 APIs (#5420)

docs/concepts/workloads/controllers/statefulset.md

@@ -10,10 +10,8 @@ title: StatefulSets
 ---
 
 {% capture overview %}
-**StatefulSets are a beta feature in 1.7. This feature replaces the
-PetSets feature from 1.4. Users of PetSets are referred to the 1.5
-[Upgrade Guide](/docs/tasks/manage-stateful-set/upgrade-pet-set-to-stateful-set/)
-for further information on how to upgrade existing PetSets to StatefulSets.**
+**StatefulSet is the workload API object used to manage stateful applications. 
+StatefulSets are beta in 1.8.**
 
 {% include templates/glossary/snippet.md term="statefulset" length="long" %}
 {% endcapture %}
@@ -51,8 +49,7 @@ The example below demonstrates the components of a StatefulSet.
 
 * A Headless Service, named nginx, is used to control the network domain.
 * The StatefulSet, named web, has a Spec that indicates that 3 replicas of the nginx container will be launched in unique Pods.
-* The volumeClaimTemplates will provide stable storage using [PersistentVolumes](/docs/concepts/storage/volumes/) provisioned by a
- PersistentVolume Provisioner.
+* The volumeClaimTemplates will provide stable storage using [PersistentVolumes](/docs/concepts/storage/volumes/) provisioned by a PersistentVolume Provisioner.
 
 ```yaml
 apiVersion: v1
@@ -69,17 +66,20 @@ spec:
   selector:
     app: nginx
 ---
-apiVersion: apps/v1beta1
+apiVersion: apps/v1beta2
 kind: StatefulSet
 metadata:
   name: web
 spec:
+  selector:
+    matchLabels:
+      app: nginx # has to match .spec.template.metadata.labels
   serviceName: "nginx"
-  replicas: 3
+  replicas: 3 # by default is 1
   template:
     metadata:
       labels:
-        app: nginx
+        app: nginx # has to match .spec.selector.matchLabels
     spec:
       terminationGracePeriodSeconds: 10
       containers:
@@ -102,6 +102,9 @@ spec:
           storage: 1Gi
 ```
 
+## Pod Selector
+You must set the `spec.selector` field of a StatefulSet to match the labels of its `.spec.template.metadata.labels`. Prior to Kubernetes 1.8, the `spec.selector` field was defaulted when omitted. In 1.8 and later versions, failing to specify a matching Pod Selector will result in a validation error during StatefulSet creation.
+
 ## Pod Identity
 StatefulSet Pods have a unique identity that is comprised of an ordinal, a
 stable network identity, and stable storage. The identity sticks to the Pod,