Warn about updating identity collection

Author: Daniel Harris

articles/batch/managed-identity-pools.md

@@ -101,6 +101,10 @@ var pool = await managementClient.Pool.CreateWithHttpMessagesAsync(
     cancellationToken: default(CancellationToken)).ConfigureAwait(false);
 ```
 
+> [!IMPORTANT]
+> Managed identities are not updated on existing VMs once a pool has been started. It is recommended to scale the pool down to zero before modifying the identity collection to ensure all VMs
+> have the same set of identities assigned.
+
 ## Use user-assigned managed identities in Batch nodes
 
 Many Azure Batch functions that access other Azure resources directly on the compute nodes, such as Azure Storage or