Add standard docs text to abstract base classes

src/ol/geom/simplegeometry.js

@@ -11,7 +11,8 @@ goog.require('ol.geom.flat.transform');
 
 /**
  * @classdesc
- * Abstract base class for geometry objects
+ * Abstract base class; normally only used for creating subclasses and not
+ * instantiated in apps.
  *
  * @constructor
  * @extends {ol.geom.Geometry}