Clean up wording of client usage expectations

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/plugin/analysis/resolver_provider.dart';
 import 'package:analysis_server/src/server/driver.dart';
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:plugin/plugin.dart';
 
 /**
- * An object that can be used to start an analysis server.
+ * An object that can be used to start an analysis server. This class exists so
+ * that clients can configure an analysis server before starting it.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 abstract class ServerStarter {
   /**
    * Initialize a newly created starter to start up an analysis server.
    */
   factory ServerStarter() = Driver;
 
   /**
    * Set the instrumentation [server] that is to be used by the analysis server.
    */
   void set instrumentationServer(InstrumentationServer server);
 
   /**
    * Set the package resolver provider used to override the way package URI's
    * are resolved in some contexts. The provider should return `null` if the
    * default package resolution scheme should be used instead.
    */
   void set packageResolverProvider(ResolverProvider provider);
 
   /**
    * 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);
 }