Add flat tree

Author: josepharhar

source

@@ -57121,7 +57121,7 @@ interface <dfn>HTMLDetailsElement</dfn> : <span>HTMLElement</span> {
      </li>
 
      <li><p>Otherwise, set <var>currentNode</var> to the parent node of
-     <var>currentNode</var>.</p></li>.
+     <var>currentNode</var> within the <span>flat tree</span>.</p></li>.
     </ol>
    </li>
   </ol>