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/smartcardManager.js

@@ -18,6 +18,9 @@ const SmartcardTokenIface = `
 
 let _smartcardManager = null;
 
+/**
+ * @returns {SmartcardManager}
+ */
 function getSmartcardManager() {
     if (_smartcardManager == null)
         _smartcardManager = new SmartcardManager();