js: Add JSDoc to exported functions and fix incorrect JSDoc formatting

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1499>

js/misc/parentalControlsManager.js

@@ -39,6 +39,9 @@ if (HAVE_MALCONTENT) {
 
 let _singleton = null;
 
+/**
+ * @returns {ParentalControlsManager}
+ */
 function getDefault() {
     if (_singleton === null)
         _singleton = new ParentalControlsManager();