Improve the documentation of extension points

pkg/analysis_server/lib/starter.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library driver;
 
 import 'package:analysis_server/src/context_manager.dart';
 import 'package:analysis_server/src/server/driver.dart';
 import 'package:analysis_server/uri/resolver_provider.dart';
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:plugin/plugin.dart';
 
 /**
  * An object that can be used to start an analysis server.
+ *
+ * Clients are not expected to subtype this class.
  */
 abstract class ServerStarter {
   /**
    * Initialize a newly created starter to start up an analysis server.
    */
   factory ServerStarter() = Driver;
 
   /**
    * Set the context manager used to create analysis contexts within each of the
    * analysis roots.
@@ skipping 16 matching lines @@
   /**
    * Set the [plugins] that are defined outside the analysis_server package.
    */
   void set userDefinedPlugins(List<Plugin> plugins);
 
   /**
    * Use the given command-line [arguments] to start this server.
    */
   void start(List<String> arguments);
 }