Explicitly mention alpha status of snapshot APIs. (#16525)

Also added a section about creating volumes from snapshots.

content/en/docs/concepts/storage/volume-snapshots.md

@@ -11,6 +11,7 @@ weight: 20
 
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+{{< feature-state for_k8s_version="v1.12" state="alpha" >}}
 This document describes the current state of `VolumeSnapshots` in Kubernetes. Familiarity with [persistent volumes](/docs/concepts/storage/persistent-volumes/) is suggested.
 
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@@ -129,4 +130,12 @@ using the attribute `snapshotClassName`.
 Only VolumeSnapshotContents of the requested class, ones with the same `snapshotClassName`
 as the VolumeSnapshot, can be bound to the VolumeSnapshot.
 
+## Provisioning Volumes from Snapshots
+
+You can provision a new volume, pre-populated with data from a snapshot, by using
+the *dataSource* field in the `PersistentVolumeClaim` object.
+
+For more details, see
+[Volume Snapshot and Restore Volume from Snapshot](/docs/concepts/storage/persistent-volumes/#volume-snapshot-and-restore-volume-from-snapshot-support).
+
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