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/windowManager.js

@@ -161,6 +161,9 @@ class WindowDimmer extends Clutter.BrightnessContrastEffect {
     }
 });
 
+/**
+ * @param {Meta.WindowActor} actor
+ */
 function getWindowDimmer(actor) {
     let effect = actor.get_effect(WINDOW_DIMMER_EFFECT_NAME);