Add comments for RoutingConfig

Author: voidzcy

xds/src/main/java/io/grpc/xds/XdsNameResolver.java

@@ -566,6 +566,9 @@ public void onResourceDoesNotExist(String resourceName) {
     }
   }
 
+  /**
+   * Grouping of the list of usable routes and their corresponding fallback timeout value.
+   */
   private static class RoutingConfig {
     private long fallbackTimeoutNano;
     private List<Route> routes;