8272148: JDesktopPane:getComponentCount() returns one extra than expe…

…cted with GTKLookAndFeel

Reviewed-by: prr, serb

src/java.desktop/share/classes/javax/swing/JComponent.java

@@ -181,6 +181,16 @@
  * need a specific value for a particular property you should
  * explicitly set it.
  * <p>
+ * A <code>JComponent</code> may contain any number of default or initial
+ * components as children. This behaviour may change according to look and
+ * feel, therefore a <code>JComponent</code> may contain some default or
+ * initial components as children for a particular Look and Feel, whereas it
+ * may not do so for some other Look and Feel. Within a particular Look and
+ * Feel also, this behaviour may change depending upon the configuration
+ * properties of the <code>JComponent</code>. In summary, it is not valid
+ * to assume a JComponent has no children just because the application
+ * did not directly add them.
+ * <p>
  * In release 1.4, the focus subsystem was rearchitected.
  * For more information, see
  * <a href="https://docs.oracle.com/javase/tutorial/uiswing/misc/focus.html">