Added comment to clarify necessity of VS retry policy

Author: ericdbishop

networking/v1alpha3/destination_rule.pb.go

@@ -647,6 +647,10 @@ message ConnectionPoolSettings {
 
     // The amount of concurrent retries allowed in the case of failure, a
     // percentage of the active requests.
+    //
+    // NOTE: Including a retry budget requires a retry policy to exist within
+    // the [VirtualService](https://istio.io/latest/docs/reference/config/networking/virtual-service/#HTTPRetry) resource
+    // routing to this destination, in order for failed requests to be retried.
     RetryBudget retry_budget = 9;
   };