Improve the documentation of extension points

pkg/analysis_server/lib/plugin/analyzed_files.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 set of files being analyzed by the
  * analysis server.
  *
  * Plugins can register a function that takes a [File] and returns a [bool]
  * indicating whether the plugin is interested in having that file be analyzed.
  * The analysis server will invoke the contributed functions and analyze the
  * file if at least one of the functions returns `true`. (The server is not
  * required to invoke every function with every file.)
+ *
+ * If a plugin is interested in analyzing a certain kind of files, it needs to
+ * ensure that files of that kind will be analyzed. It should register a
+ * function by including code like the following in the plugin's
+ * registerExtensions method:
+ *
+ *     @override
+ *     void registerExtensions(RegisterExtension registerExtension) {
+ *       ...
+ *       registerExtension(
+ *           ANALYZE_FILE_EXTENSION_POINT_ID,
+ *           (File file) => file.path.endsWith(...));
+ *       ...
+ *     }
  */
 library analysis_server.plugin.analyzed_files;
 
 import 'package:analysis_server/src/plugin/server_plugin.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:plugin/plugin.dart';
 
 /**
  * The identifier of the extension point that allows plugins to register
  * functions that can cause files to be analyzed. The object used as an
  * extension must be a [ShouldAnalyzeFile] function.
  */
 final String ANALYZE_FILE_EXTENSION_POINT_ID = Plugin.join(
     ServerPlugin.UNIQUE_IDENTIFIER, ServerPlugin.ANALYZE_FILE_EXTENSION_POINT);
 
 /**
  * A function that returns `true` if the given [file] should be analyzed.
  */
 typedef bool ShouldAnalyzeFile(File file);