Add gov cloud regions are only supported in PS/CLI.

Author: wmgries

articles/storage/files/understanding-billing.md

@@ -137,9 +137,11 @@ Currently, these SKUs are generally available in a limited subset of regions:
 - Korea South
 - East US 2
 - Central US
-- US Gov Virginia
-- US Gov Arizona
-- US Gov Texas
+- US Gov Virginia*
+- US Gov Arizona*
+- US Gov Texas*
+
+Storage accounts in regions flagged with an asterisks (*) can only be created using Azure PowerShell or Azure CLI. See [create a storage account](./storage-how-to-create-file-share.md) for more information.
 
 ### Provisioned v2 provisioning detail
 When you create a provisioned v2 file share, you specify the provisioned capacity for the file share in terms of storage, IOPS, and throughput. File shares are limited based on the following attributes: