Document change events properly

With this change, the generic 'change' event is properly documented, as
all other events. It is no longer necessary to annotate `@fires change` for
every ol.Observable subclass.

src/ol/observable.js

@@ -16,6 +16,7 @@ goog.require('goog.events.EventType');
  *
  * @constructor
  * @extends {goog.events.EventTarget}
+ * @fires change
  * @suppress {checkStructDictInheritance}
  * @struct
  * @api stable
@@ -46,7 +47,6 @@ ol.Observable.unByKey = function(key) {
 
 /**
  * Increases the revision counter and dispatches a 'change' event.
- * @fires change
  * @api
  */
 ol.Observable.prototype.changed = function() {
@@ -55,6 +55,13 @@ ol.Observable.prototype.changed = function() {
 };
 
 
+/**
+ * Triggered when the revision counter is increased.
+ * @event change
+ * @api
+ */
+
+
 /**
  * @return {number} Revision.
  * @api