Postpone Deletion of a Persistent Volume Claim in case It Is Used by a Pod (#6415)

* Postpone Deletion of a Persistent Volume Claim in case It Is Used by a Pod

A new feature PVC Protection was added into K8s 1.9 that's why this documentation change is needed.

* Added tag at the top of each new area.

* Fix typo

* Fix: switched on in (all kubelets) -> (all K8s components).

* Added link to admission controller

* Moved PVC Protection configuration into Before you begin section.

* Added steps how to verify PVC Protection feature.

* Fixes for admission controller plugin description and for PVC Protection description in PVC lifecycle.

* Testing official rendering of enumerations (1., 2., 3., etc.)

* Re-write to address comments from review.

* Fixed definition when a PVC is in active use by a pod.

_data/tasks.yml

@@ -169,6 +169,7 @@ toc:
   - docs/tasks/administer-cluster/configure-multiple-schedulers.md
   - docs/tasks/administer-cluster/ip-masq-agent.md
   - docs/tasks/administer-cluster/dns-custom-nameservers.md
+  - docs/tasks/administer-cluster/pvc-protection.md
 
 - title: Federation - Run an App on Multiple Clusters
   section: