Improve the documentation of extension points

pkg/analysis_server/lib/plugin/fix.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 fix
  * contributors.
+ *
+ * Plugins can register fix contributors. The registered contributors will be
+ * used to get fixes any time a client issues an 'edit.getFixes' request.
+ *
+ * If a plugin wants to add fixes, it should implement the class
+ * [FixContributor] and then register the contributor by including code like the
+ * following in the plugin's registerExtensions method:
+ *
+ *     @override
+ *     void registerExtensions(RegisterExtension registerExtension) {
+ *       ...
+ *       registerExtension(
+ *           FIX_CONTRIBUTOR_EXTENSION_POINT_ID,
+ *           new MyFixContributor());
+ *       ...
+ *     }
  */
 library analysis_server.plugin.fix;
 
 import 'package:analysis_server/edit/fix/fix_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 fix
  * contributors. The object used as an extension must be a [FixContributor].
  */
 final String FIX_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
     ServerPlugin.UNIQUE_IDENTIFIER,
     ServerPlugin.FIX_CONTRIBUTOR_EXTENSION_POINT);