Clarify that StatefulSet is stable in 1.9 (kubernetes#6697)

Author: dennis-benzinger-hybris

docs/concepts/workloads/controllers/statefulset.md

@@ -41,7 +41,7 @@ provides a set of stateless replicas. Controllers such as
 
 ## Limitations
 
-* StatefulSet is a beta resource, not available in any Kubernetes release prior to 1.5.
+* StatefulSet was a beta resource prior to 1.9 and not available in any Kubernetes release prior to 1.5.
 * As with all alpha/beta resources, you can disable StatefulSet through the `--runtime-config` option passed to the apiserver.
 * The storage for a given Pod must either be provisioned by a [PersistentVolume Provisioner](https://github.com/kubernetes/examples/tree/{{page.githubbranch}}/staging/persistent-volume-provisioning/README.md) based on the requested `storage class`, or pre-provisioned by an admin.
 * Deleting and/or scaling a StatefulSet down will *not* delete the volumes associated with the StatefulSet. This is done to ensure data safety, which is generally more valuable than an automatic purge of all related StatefulSet resources.