iothread: document about why we need explicit aio_poll()

    qemu_sem_post(&iothread->init_done_sem);

    while (iothread->running) {

        /*

         * Note: from functional-wise the g_main_loop_run() below can

         * already cover the aio_poll() events, but we can't run the

         * main loop unconditionally because explicit aio_poll() here

         * is faster than g_main_loop_run() when we do not need the

         * gcontext at all (e.g., pure block layer iothreads).  In

         * other words, when we want to run the gcontext with the

         * iothread we need to pay some performance for functionality.

         */

        aio_poll(iothread->ctx, true);

        /*