Finalize pvc support (#5651)

* pvc support

* comments

* fixes

* fix

docs/serving/services/storage.md

@@ -1,12 +1,36 @@
 # Volume Support for Knative services
 
-By default Serving supports the mounting the [volume types](https://kubernetes.io/docs/concepts/storage/volumes): `emptyDir`, `secret`, `configMap` and `projected`. [PersistentVolumes](https://kubernetes.io/docs/concepts/storage/persistent-volumes/) are supported but require a [feature flag](../configuration/feature-flags.md) to be enabled.
+You can provide data storage for Knative Services by configuring different volumes types.
+Serving supports mounting the [volume types](https://kubernetes.io/docs/concepts/storage/volumes): `emptyDir`, `secret`, `configMap` and `projected`.
+[PersistentVolumes](https://kubernetes.io/docs/concepts/storage/persistent-volumes/) are supported but require a [feature flag](../configuration/feature-flags.md) to be enabled.
 
 !!! warning
+
     Mounting large volumes may add considerable overhead to the application's start up time.
 
 
-Here is an example of using a persistent volume claim with a Knative Service:
+Bellow there is an example of using a persistent volume claim with a Knative Service.
+
+## Prerequisites
+
+Before you can configure PVCs for a Service, this feature must be enabled in the `config-features` ConfigMap as follows:
+
+```
+kubectl patch --namespace knative-serving configmap/config-features \
+ --type merge \
+ --patch '{"data":{"kubernetes.podspec-persistent-volume-claim": "enabled", "kubernetes.podspec-persistent-volume-write": "enabled"}}'
+```
+
+* The `kubernetes.podspec-persistent-volume-claim` extension controls whether persistent volumes (PVs) can be used with Knative Serving.
+* The `kubernetes.podspec-persistent-volume-write` extension controls whether PVs are available to Knative Serving with the write access.
+
+!!! note
+
+    If you have installed Serving via the Knative operator then you need to set the above feature flags **only** at the corresponding Serving CR.
+
+## Procedure
+
+* Modify the PVC configuration for your Service:
 
 ```yaml
 apiVersion: serving.knative.dev/v1
@@ -16,8 +40,8 @@ spec:
   template:
     spec:
       containers:
-          ...
-          volumeMounts:
+        ...
+        volumeMounts:
             - mountPath: /data
               name: mydata
               readOnly: false
@@ -37,4 +61,4 @@ spec:
   resources:
     requests:
       storage: 1Gi
-```
+```