Issue #1908192 by mitron: Document what happens in drupal_get_filename() if the file is not found

core/includes/bootstrap.inc

@@ -859,7 +859,7 @@ function drupal_settings_initialize() {
  *   than by consulting the database.
  *
  * @return
- *   The filename of the requested item.
+ *   The filename of the requested item or NULL if the item is not found.
  */
 function drupal_get_filename($type, $name, $filename = NULL) {
   // The location of files will not change during the request, so do not use