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/background.js

@@ -229,6 +229,9 @@ var BackgroundCache = class BackgroundCache extends Signals.EventEmitter {
     }
 };
 
+/**
+ * @returns {BackgroundCache}
+ */
 function getBackgroundCache() {
     if (!_backgroundCache)
         _backgroundCache = new BackgroundCache();