Improve the documentation of extension points

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