Update i18n pages to include predefined messages (e.g. @@bidi_dir)....

chrome/common/extensions/docs/static/i18n-messages.html

 <div id="pageData-name" class="pageData">Formats: Locale-Specific Messages</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 Each internationalized extension has at least one
 file named <code>messages.json</code>
 that provides locale-specific strings for the extension.
 This page describes the format of <code>messages.json</code> files.
 For information on how to internationalize and localize your extension,
 see the <a href="i18n.html">Internationalization</a> page.
@@ skipping 91 matching lines @@
 The name is a case-insensitive key
 that lets you retrieve the localized message text.
 The name can include the following characters:
 </p>
 
 <ul>
   <li> A-Z </li>
   <li> a-z </li>
   <li> 0-9 </li>
   <li> _ (underscore) </li>
+  <li> @ </li>
 </ul>
 
+<p class="note">
+<b>Note:</b>
+Don't define names that begin with "@@".
+Those names are reserved for
+<a href="i18n.html#overview-predefined">predefined messages</a>.
+</p>
+
 <p>
 Here are three examples of names,
 taken from the <a href="#example">Example</a> section:
 </p>
 
 <pre>
 "prompt_for_name": {
   ...
 },
 "hello": {
@@ skipping 192 matching lines @@
   },
   "user": {
     "content": "$1",
     "example": "Cira"
   }
 }
 </pre>