Specification of JComponent::updateUI should document that the default implementation does nothing

Summary

It's unclear from the spec that the original implementation of the method does nothing in https://docs.oracle.com/en/java/javase/15/docs/api/java.desktop/javax/swing/JComponent.html#updateUI()

Problem

The current state of specification for https://docs.oracle.com/en/java/javase/15/docs/api/java.desktop/javax/swing/JComponent.html#updateUI() is unclear that the original implementation of the method does nothing. This deserves to be mentioned explicitly.

Solution

javadoc updated to mention that the base method does not do anything via @implSpec tag.

Specification

    /**
-     * Resets the UI property to a value from the current look and feel.
-     * <code>JComponent</code> subclasses must override this method
+     * This method is called to update the UI property to a value from the
+     * current look and feel.
+     * {@code JComponent} subclasses must override this method
      * like this:
      * <pre>
      *   public void updateUI() {
      *      setUI((SliderUI)UIManager.getUI(this);
      *   }
      *  </pre>
      *
+     * @implSpec The default implementation of this method does nothing.
+     *
      * @see #setUI
      * @see UIManager#getLookAndFeel
      * @see UIManager#getUI
      */
     public void updateUI() {}