libbinder: doc RpcServer::join/shutdown behavior

     * If a client needs to actively terminate join, call shutdown() in a separate thread.

     *

     * At any given point, there can only be one thread calling join().

     *

     * Warning: if shutdown is called, this will return while the shutdown is

     * still occurring. To ensure that the service is fully shutdown, you might

     * want to call shutdown after 'join' returns.

     */

    void join();

     * (e.g. no join() is running). Will wait for the server to be fully

     * shutdown.

     *

     * TODO(b/185167543): wait for sessions to shutdown as well

     * Warning: this will hang if it is called from its own thread.

     */

    [[nodiscard]] bool shutdown();