Update mock-response-policy.md

Fixes https://github.com/MicrosoftDocs/azure-docs/issues/103071

Author: dlepow

articles/api-management/mock-response-policy.md

@@ -36,6 +36,10 @@ The `mock-response` policy, as the name implies, is used to mock APIs and operat
 - [**Policy scopes:**](./api-management-howto-policies.md#scopes) global, product, API, operation
 -  [**Gateways:**](api-management-gateways-overview.md) dedicated, consumption, self-hosted
 
+### Usage notes
+
+-  [Policy expressions](api-management-policy-expressions.md) can't be used in attribute values for this policy.
+
 ## Examples
 
 ```xml
@@ -50,4 +54,4 @@ The `mock-response` policy, as the name implies, is used to mock APIs and operat
 
 * [API Management advanced policies](api-management-advanced-policies.md)
 
-[!INCLUDE [api-management-policy-ref-next-steps](../../includes/api-management-policy-ref-next-steps.md)]
+[!INCLUDE [api-management-policy-ref-next-steps](../../includes/api-management-policy-ref-next-steps.md)]