Document that layer options are set as object properties

src/ol/layer/vectorlayer.js

@@ -17,6 +17,9 @@ ol.layer.VectorProperty = {
 /**
  * @classdesc
  * Vector data that is rendered client-side.
+ * 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.Layer}