Add standard docs text to abstract base classes

src/ol/observable.js

@@ -8,7 +8,8 @@ goog.require('goog.events.EventType');
 
 /**
  * @classdesc
- * Abstract base class.
+ * Abstract base class; normally only used for creating subclasses and not
+ * instantiated in apps.
  * An event target providing convenient methods for listener registration
  * and unregistration. A generic `change` event is always available through
  * {@link ol.Observable#dispatchChangeEvent}.