Documented requirement for Cache Storage accounts

Added clarification about the requirements for cache storage accounts.  This requirement was verified with MS Support (see SR 2303080040008036).

Author: rcabr

articles/site-recovery/azure-to-azure-support-matrix.md

@@ -72,6 +72,7 @@ Subscription  | Can be different from source virtual machines | Cache storage ac
 Azure Storage firewalls for virtual networks  | Supported | If you're using firewall enabled cache storage account or target storage account, ensure you ['Allow trusted Microsoft services'](../storage/common/storage-network-security.md#exceptions).<br></br>Also, ensure that you allow access to at least one subnet of source Vnet.<br></br>Note: Don't restrict virtual network access to your storage accounts used for Site Recovery. You should allow access from 'All networks'.
 Soft delete | Not supported | Soft delete isn't supported because once it is enabled on cache storage account, it increases cost. Azure Site Recovery performs frequent creates/deletes of log files while replicating causing costs to increase.
 Encryption at rest (CMK) | Supported | Storage account encryption can be configured with customer managed keys (CMK)
+Managed identity | Not supported | The cached storage account must allow shared key access and Shared Access Signatures (SAS) signed by the shared key.
 
 The following table lists the limits in terms of number of disks that can replicate to a single storage account.