Adding block volumeMode documentation for local volumes. (#7531)

Code review comments.

Changed property to field.

Address tech review comment.

docs/concepts/storage/persistent-volumes.md

@@ -473,7 +473,7 @@ spec:
 
 ## Raw Block Volume Support
 
-Static provisioning support for Raw Block Volumes is included as an alpha feature for v1.9. With this change are some new API fields that need to be used to facilitate this functionality. Currently, Fibre Channel is the only supported plugin for this feature.
+Static provisioning support for Raw Block Volumes is included as an alpha feature for v1.9. With this change are some new API fields that need to be used to facilitate this functionality. Kubernetes v1.10 supports only Fibre Channel and Local Volume plugins for this feature.
 
 ### Persistent Volumes using a Raw Block Volume
 ```yaml

docs/concepts/storage/volumes.md

@@ -543,6 +543,8 @@ metadata:
 spec:
   capacity:
     storage: 100Gi
+  # volumeMode field requires BlockVolume Alpha feature gate to be enabled.
+  volumeMode: Filesystem
   accessModes:
   - ReadWriteOnce
   persistentVolumeReclaimPolicy: Delete
@@ -563,6 +565,10 @@ PersistentVolume `nodeAffinity` is required when using local volumes. It enables
 the Kubernetes scheduler to correctly schedule pods using local volumes to the
 correct node.
 
+PersistentVolume `volumeMode` can now be set to "Block" (instead of the default
+value "Filesystem") to expose the local volume as a raw block device. The
+`volumeMode` field requires `BlockVolume` Alpha feature gate to be enabled.
+
 When using local volumes, it is recommended to create a StorageClass with
 `volumeBindingMode` set to `WaitForFirstConsumer`. See the
 [example](storage-classes.md#local). Delaying volume binding ensures