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

pkg/intl/lib/intl.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.
 
 /**
  * This library provides internationalization and localization. This includes
  * message formatting and replacement, date and number formatting and parsing,
  * and utilities for working with Bidirectional text.
  *
  * ## Installing ##
@@ skipping 238 matching lines @@
   /**
    * Format the given function with a specific [locale], given a
    * [message_function] that takes no parameters. The [message_function] can be
    * a simple message function that just returns the result of `Intl.message()`
    * it can be a wrapper around a message function that takes arguments, or it
    * can be something more complex that manipulates multiple message
    * functions.
    *
    * In either case, the purpose of this is to delay calling [message_function]
    * until the proper locale has been set. This returns the result of calling
-   * [msg_function], which could be of an arbitrary type.
+   * [message_function], which could be of an arbitrary type.
    */
   static withLocale(String locale, Function message_function) {
     // We have to do this silliness because Locale is not known at compile time,
     // but must be a static variable in order to be visible to the Intl.message
     // invocation.
     var oldLocale = getCurrentLocale();
     _defaultLocale = locale;
     var result = message_function();
     _defaultLocale = oldLocale;
     return result;
@@ skipping 18 matching lines @@
    * unless for some reason this gets called inside a message that resets the
    * locale.
    */
   static String getCurrentLocale() {
     if (_defaultLocale == null) _defaultLocale = systemLocale;
     return _defaultLocale;
   }
 
   toString() => "Intl($locale)";
 }