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/ibusManager.js

@@ -46,6 +46,9 @@ function _checkIBusVersion(requiredMajor, requiredMinor, requiredMicro) {
         `but required is ${requiredMajor}.${requiredMinor}.${requiredMicro}`);
 }
 
+/**
+ * @returns {IBusManager}
+ */
 function getIBusManager() {
     if (_ibusManager == null)
         _ibusManager = new IBusManager();