Correct/improve dragbox docs

src/ol/interaction/interaction.js

@@ -20,6 +20,8 @@ goog.require('ol.easing');
  * For example, {@link ol.interaction.KeyboardZoom} is functionally the same as
  * {@link ol.control.Zoom}, but triggered by a keyboard event not a button
  * element event.
+ * Although interactions do not have a DOM element, some of them do render
+ * vectors and so are visible on the screen.
  *
  * @constructor
  * @extends {ol.Observable}