Better document which events are fired

The map does not fire ol.render.Event#render. Change events are
now only listed for classes that fire them, and a description
about when they are fired is added.

src/ol/layer/layer.js

@@ -13,6 +13,7 @@ goog.require('ol.source.Source');
  * @constructor
  * @extends {ol.layer.Base}
  * @fires ol.render.Event
+ * @fires change Triggered when the state of the source changes.
  * @param {olx.layer.LayerOptions} options Layer options.
  * @todo api
  */

src/ol/layer/layergroup.js

@@ -27,6 +27,8 @@ ol.layer.GroupProperty = {
 /**
  * @constructor
  * @extends {ol.layer.Base}
+ * @fires change Triggered when the state of the source of any of the layers of
+ *     this group changes
  * @param {olx.layer.GroupOptions=} opt_options Layer options.
  * @todo api
  */