Document that layer options are set as object properties

src/ol/layer/layerbase.js

@@ -44,6 +44,9 @@ ol.layer.LayerState;
  * @classdesc
  * Abstract base class; normally only used for creating subclasses and not
  * instantiated in apps.
+ * Note that with `ol.layer.Base` and all its subclasses, any property set in
+ * the options is set as a {@link ol.Object} property on the layer object, so
+ * is observable, and has get/set accessors.
  *
  * @constructor
  * @extends {ol.Object}