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

pkg/intl/lib/generate_localized.dart

 // Copyright (c) 2013, 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 provides utilities for generating localized versions of
  * messages. It does not stand alone, but expects to be given
  * TranslatedMessage objects and generate code for a particular locale
  * based on them.
  *
@@ skipping 88 matching lines @@
       .map((translation) => translation.originalMessage.name)
       .map((name) => "    \"$name\" : $name");
   result.write(entries.join(",\n"));
   result.write("\n  };\n}");
 
   var output = new File(path.join(targetDir, "messages_$locale.dart"));
   output.writeAsStringSync(result.toString());
 }
 
 /**
- * This returns the mostly constant string used in [generated] for the
+ * This returns the mostly constant string used in
+ * [generateIndividualMessageFile] for the
  * beginning of the file, parameterized by [locale].
  */
 String prologue(String locale) => """
 /**
  * DO NOT EDIT. This is code generated via pkg/intl/generate_localized.dart
  * This is a library that provides messages for a $locale locale. All the
  * messages from the main program should be duplicated here with the same
  * function name.
  */
 
@@ skipping 45 matching lines @@
 library messages_all;
 
 import 'dart:async';
 import 'package:$intlImportPath/message_lookup_by_library.dart';
 import 'package:$intlImportPath/src/intl_helpers.dart';
 import 'package:$intlImportPath/intl.dart';
 
 """;
 
 /**
- * Constant string used in [generateMainImportfile] as the end of the file.
+ * Constant string used in [generateMainImportFile] as the end of the file.
  */
 const closing = """
     default: return null;
   }
 }
 
 /** User programs should call this before using [localeName] for messages.*/
 initializeMessages(localeName) {
   initializeInternalMessageLookup(() => new CompositeMessageLookup());
   messageLookup.addLocale(localeName, _findGeneratedMessagesFor);
   return new Future.value();
 }
 
 MessageLookupByLibrary _findGeneratedMessagesFor(locale) {
   var actualLocale = Intl.verifiedLocale(locale, (x) => _findExact(x) != null);
   if (actualLocale == null) return null;
   return _findExact(actualLocale);
 }
 """;