Basically, any collaborator in the scenegraph could implement skinnable, not only controls. Actually, there is at least one (PopupControl) that isn't. 

So all methods in the interface Skinnable should specify _Skinnable_ (vs _Control_ as they do now)

for instance:

    /**
     * Sets the skin that will render this {@link Control}
     * @param value the skin value for this control
     */
    public void setSkin(Skin<?> value);

should be changed to (modulo doc conventions, not entirely certain about lower/upper casing nor self-references?)

    /**
     * Sets the skin that will render this skinnable.
     * @param value the skin value for this skinnable
     */
    public void setSkin(Skin<?> value);

Apart from that, the description of Skinnable needs some love - it's an implementation note, not very expressive and stating the obvious:

    The Skinnable interface is implemented by the Control class, and therefore is implemented by all Control implementations.
 
Should be changed to describe _what_ it is for, maybe somehow the inverse of Skin: 

   A Skinnable delegates its visual representation to its {@link Skin}. ...

Without updating the description, the change might be small enough to go into JDK-8252389 (but as discussed its PR: if we want to change the description as well, we'll need a separate issue anyway)