Fixed gh-467 so that the XML build of fields will work correctly with the new groupedlist field type. Added placeholders to class comments.

admin/helpers/compiler/c_Fields.php

@@ -1800,7 +1800,7 @@ class Fields extends Structure
 
 								foreach ($grouped_[$key_][$_id] as $option_)
 								{
-									$groupOptionXML = $groupXML->fieldXML->addChild('option');
+									$groupOptionXML = $groupXML->addChild('option');
 									$groupOptionXML->addAttribute('value', $option_['value']);
 									$groupOptionXML[] = $option_['text'];
 								}