Pass null as forEachFeatureAtPixel layer arg for unmanaged layers

src/ol/layer/layer.js

@@ -20,6 +20,11 @@ goog.require('ol.source.State');
  * Layers group together those properties that pertain to how the data is to be
  * displayed, irrespective of the source of that data.
  *
+ * Layers are usually added to a map with {@link ol.Map#addLayer}. Components
+ * like {@link ol.interaction.Select} use unmanaged layers internally. These
+ * unmanaged layers are associated with the map using
+ * {@link ol.layer.Layer#setMap} instead.
+ *
  * A generic `change` event is fired when the state of the source changes.
  *
  * @constructor