Add information for account SAS

Author: pauljewellmsft

articles/storage/common/storage-account-sas-create-dotnet.md

@@ -21,6 +21,15 @@ ms.custom: devx-track-csharp
 
 This article shows how to use the storage account key to create an account SAS with the [Azure Storage client library for .NET](/dotnet/api/overview/azure/storage).
 
+## About the account SAS
+
+An account SAS is created at the level of the storage account. By creating an account SAS, you can:
+
+- Delegate access to service-level operations that aren't currently available with a service-specific SAS, such as [Get Blob Service Properties](/rest/api/storageservices/get-blob-service-properties), [Set Blob Service Properties](/rest/api/storageservices/set-blob-service-properties) and [Get Blob Service Stats](/rest/api/storageservices/get-blob-service-stats).
+- Delegate access to more than one service in a storage account at a time. For example, you can delegate access to resources in both Azure Blob Storage and Azure Files by using an account SAS.
+
+Stored access policies are not supported for an account SAS.
+
 ## Create an account SAS
 
 An account SAS is signed with the account access key. Use the [StorageSharedKeyCredential](/dotnet/api/azure.storage.storagesharedkeycredential) class to create the credential that is used to sign the SAS. Next, create a new [AccountSasBuilder](/dotnet/api/azure.storage.sas.accountsasbuilder) object and call the [ToSasQueryParameters](/dotnet/api/azure.storage.sas.accountsasbuilder.tosasqueryparameters) to get the SAS token string.