Improve and standardise event docs

src/ol/events/condition.jsdoc

@@ -1,4 +1,15 @@
 /**
+ * Applications do not normally create event instances. They register (and
+ * unregister) event listener functions, which, when called by the library as
+ * the result of an event being dispatched, are passed event instances as their
+ * first argument. Listeners can be registered and unregistered on all objects
+ * descending from {@link ol.Observable}. All event instances have a `target`
+ * property, which corresponds to the object on which the event was dispatched.
+ * By default, `this` within the listener also refers to the target, though
+ * this can be configured in the listener registration function.
+ * Some classes have their own event type, which return additional
+ * properties; see the specific event class page for details.
+ *
  * @namespace ol.events
  */