Document that ForkJoinWorkerThreadFactory.newThread can return null to reject request

ThreadFactory.newThread is documented to return null if the request is rejected. The very similar ForkJoinWorkerThreadFactory should do likewise

     * @return constructed thread, or {@code null} if the request to
     * create a thread is rejected
     */
    Thread newThread(Runnable r);

         * @return the new worker thread, or {@code null} if the request
         * to create a thread is rejected
         * @throws NullPointerException if the pool is null
         */
        public ForkJoinWorkerThread newThread(ForkJoinPool pool);