Create a custom control by extending the Control class.  For example:

public class CustomControl extends javafx.scene.control.Control
{
}

Now run: javadoc CustomControl.java -link http://docs.oracle.com/javase/8/javafx/api/

You will notice that the generated CustomControl.html file links to impl_* methods of Control, Region, Parent and Node.

Developers of control libraries would not want their documentation cluttered with links to impl_* methods (which are already broken since JavaFX does not document them).