add purview msi disclaimer

Author: helizabethwhite

articles/purview/manage-credentials.md

@@ -62,7 +62,7 @@ Before you can create a Credential, first associate one or more of your existing
 
 4. In the **Secrets permissions** dropdown, select **Get** and **List** permissions.
 
-5. For **Select principal**, choose the Purview managed identity.
+5. For **Select principal**, choose the Purview managed identity. You can search for the Purview MSI using either the Purview instance name **or** the managed identity application ID. We do not currently support compound identities (managed identity name + application ID).
 
    :::image type="content" source="media/manage-credentials/add-access-policy.png" alt-text="Add access policy":::