Document that layer options are set as object properties

src/ol/layer/heatmaplayer.js

@@ -23,6 +23,9 @@ ol.layer.HeatmapLayerProperty = {
 /**
  * @classdesc
  * Layer for rendering vector data as a heatmap.
+ * Note that any property set in the options is set as a {@link ol.Object}
+ * property on the layer object; for example, setting `title: 'My Title'` in the
+ * options means that `title` is observable, and has get/set accessors.
  *
  * @constructor
  * @extends {ol.layer.Vector}