Explain better than the en_US locale is always available, fix other doc glitches.

pkg/intl/lib/message_lookup_by_library.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Message/plural format library with locale support. This can have different
  * implementations based on the mechanism for finding the localized versions
  * of messages. This version expects them to be in a library named e.g.
- * 'messages_en_US'. The prefix is set in the [initializeMessages] call, which
+ * 'messages_en_US'. The prefix is set in the "initializeMessages" call, which
  * must be made for a locale before any lookups can be done.
  *
  * See Intl class comment or `tests/message_format_test.dart` for more examples.
  */
 library message_lookup_by_library;
 
 import 'dart:async';
 import 'intl.dart';
 import 'src/intl_helpers.dart';
 
@@ skipping 105 matching lines @@
    * Subclasses should override this to return a list of their message
    * functions.
    */
   Map<String, Function> get messages;
 
   /** Subclasses should override this to return their locale, e.g. 'en_US' */
   String get localeName;
 
   toString() => localeName;
 }