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/status/keyboard.js

@@ -799,6 +799,9 @@ var InputSourceManager = class extends Signals.EventEmitter {
 
 let _inputSourceManager = null;
 
+/**
+ * @returns {InputSourceManager}
+ */
 function getInputSourceManager() {
     if (_inputSourceManager == null)
         _inputSourceManager = new InputSourceManager();