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

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

js/ui/appFavorites.js

@@ -206,6 +206,10 @@ class AppFavorites extends Signals.EventEmitter {
 }
 
 var appFavoritesInstance = null;
+
+/**
+ * @returns {AppFavorites}
+ */
 function getAppFavorites() {
     if (appFavoritesInstance == null)
         appFavoritesInstance = new AppFavorites();