RLS config: update comments on max_age and stale_age

grpc/lookup/v1/rls_config.proto

@@ -188,14 +188,16 @@ message RouteLookupConfig {
 
   // How long are responses valid for (like HTTP Cache-Control).
   // If omitted or zero, the longest valid cache time is used.
-  // This value is clamped to 5 minutes to avoid unflushable bad responses.
+  // This value is clamped to 5 minutes to avoid unflushable bad responses,
+  // unless stale_age is specified.
   google.protobuf.Duration max_age = 5;
 
   // After a response has been in the client cache for this amount of time
   // and is re-requested, start an asynchronous RPC to re-validate it.
   // This value should be less than max_age by at least the length of a
   // typical RTT to the Route Lookup Service to fully mask the RTT latency.
   // If omitted, keys are only re-requested after they have expired.
+  // This value is clamped to 5 minutes.
   google.protobuf.Duration stale_age = 6;
 
   // Rough indicator of amount of memory to use for the client cache.  Some of