8368576: PrintJob.getGraphics() does not specify behavior after PrintJob.end()

Reviewed-by: psadhukhan, tr, serb

Author: prrace

src/java.desktop/share/classes/java/awt/PrintJob.java

@@ -46,6 +46,8 @@ protected PrintJob() {}
      * The page is sent to the printer when the graphics
      * object is disposed.  This graphics object will also implement
      * the PrintGraphics interface.
+     * If {@code PrintJob.end()} has been called, this method will
+     * return {@code null}.
      * @see PrintGraphics
      * @return the graphics context for printing the next page
      */