Improve the documentation of extension points

pkg/analysis_server/lib/plugin/navigation.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.
 
 /**
  * Support for client code that extends the analysis server by adding new
  * navigation contributors.
+ *
+ * Plugins can register navigation contributors. The registered contributors
+ * will be used to get navigation regions any time a client issues an
+ * 'analysis.getNavigation' request or the server is about to send an
+ * 'analysis.navigation' notification.
+ *
+ * If a plugin wants to add navigation regions, it should implement the class
+ * [NavigationContributor] and then register the contributor by including code
+ * like the following in the plugin's registerExtensions method:
+ *
+ *     @override
+ *     void registerExtensions(RegisterExtension registerExtension) {
+ *       ...
+ *       registerExtension(
+ *           NAVIGATION_CONTRIBUTOR_EXTENSION_POINT_ID,
+ *           new MyNavigationContributor());
+ *       ...
+ *     }
  */
 library analysis_server.plugin.navigation;
 
 import 'package:analysis_server/analysis/navigation_core.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
 import 'package:plugin/plugin.dart';
 
 /**
  * The identifier of the extension point that allows plugins to register
  * navigation contributors. The object used as an extension must be
  * a [NavigationContributor].
  */
 final String NAVIGATION_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
     ServerPlugin.UNIQUE_IDENTIFIER,
     ServerPlugin.NAVIGATION_CONTRIBUTOR_EXTENSION_POINT);