Improve class docs

Follow-up to 00603f6

lib/Pico.php

@@ -819,10 +819,11 @@ class Pico
      * Returns the raw contents of the first found 404 file when traversing
      * up from the directory the requested file is in
      *
+     * If no suitable `404.md` is found, fallback to a built-in error message.
+     *
      * @see    Pico::getRawContent()
      * @param  string $file     path to requested (but not existing) file
      * @return string           raw contents of the 404 file
-     * @throws RuntimeException thrown when no suitable 404 file is found
      */
     public function load404Content($file)
     {