JLabel and AbstractButton's imageUpdate method should be better specified

The doc for these methods currently reads:

     * This is overridden to return false if the current Icon's Image is
     * not equal to the passed in Image <code>img</code>.

But this isn't quite right as Icon's don't have an Image. Instead it should say something like:

returns true if one of this JLabel's icon or disabled icon is derived from
ImageIcon and it's Image is equal to the image passed in; false
otherwise.