Update doc of interceptor about response order requirements
diff --git a/src/objective-c/GRPCClient/GRPCInterceptor.h b/src/objective-c/GRPCClient/GRPCInterceptor.h
index 18abe54..166990f 100644
--- a/src/objective-c/GRPCClient/GRPCInterceptor.h
+++ b/src/objective-c/GRPCClient/GRPCInterceptor.h
@@ -97,6 +97,10 @@
receiveNextMessages
\endverbatim
*
+ * An interceptor must forward responses to its previous interceptor in the order of initial
+ * metadata, message(s), and trailing metadata. Forwarding responses out of this order (e.g.
+ * forwarding a message before initial metadata) is not allowed.
+ *
* Events of requests and responses are dispatched to interceptor objects using the interceptor's
* dispatch queue. The dispatch queue should be serial queue to make sure the events are processed
* in order. Interceptor implementations must derive from GRPCInterceptor class. The class makes
