Update storage-classes.md (kubernetes#7408)

Added the detail of storage class with Azure Managed Disk.

docs/concepts/storage/storage-classes.md

@@ -476,7 +476,9 @@ parameters:
   When `kind` is `shared`, all unmanaged disks are created in a few shared
   storage accounts in the same resource group as the cluster. When `kind` is
   `dedicated`, a new dedicated storage account will be created for the new
-  unmanaged disk in the same resource group as the cluster.
+  unmanaged disk in the same resource group as the cluster. When `kind` is 
+  `managed`, all managed disks are created in the same resource group as 
+  the cluster.
 
 - Premium VM can attach both Standard_LRS and Premium_LRS disks, while Standard
   VM can only attach Standard_LRS disks.