Clarifying specific client-id value for system-assigned identity

Author: nimakamoosi

articles/api-management/authentication-managed-identity-policy.md

@@ -31,7 +31,7 @@ Both system-assigned identity and any of the multiple user-assigned identities c
 | Attribute         | Description                                            | Required | Default |
 | ----------------- | ------------------------------------------------------ | -------- | ------- |
 |resource|String. The application ID of the target web API (secured resource) in Microsoft Entra ID. Policy expressions are allowed. |Yes|N/A|
-|client-id|String. The client ID of the user-assigned identity in Microsoft Entra ID. Policy expressions aren't allowed. |No|system-assigned identity|
+|client-id|String. The client ID of the user-assigned identity in Microsoft Entra ID. Policy expressions aren't allowed. |No|`""` for system-assigned identity|
 |output-token-variable-name|String. Name of the context variable that will receive token value as an object of type `string`. Policy expressions aren't allowed. |No|N/A|  
 |ignore-error|Boolean. If set to `true`, the policy pipeline continues to execute even if an access token isn't obtained.|No|`false`|