hw/display/xlnx_dp: update comments

Clarify somewhat misleading code comments.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/display/xlnx_dp.c

@@ -380,13 +380,16 @@ static inline void xlnx_dp_audio_mix_buffer(XlnxDPState *s)
 static void xlnx_dp_audio_callback(void *opaque, int avail)
 {
     /*
-     * Get some data from the DPDMA and compute these data.
-     * Then wait for QEMU's audio subsystem to call this callback.
+     * Get the individual left and right audio streams from the DPDMA,
+     * and fill the output buffer with the combined stereo audio data
+     * adjusted by the volume controls.
+     * QEMU's audio subsystem will call this callback repeatedly;
+     * we return the data from the output buffer until it is emptied,
+     * and then we will read data from the DPDMA again.
      */
     XlnxDPState *s = XLNX_DP(opaque);
     size_t written = 0;
 
-    /* If there are already some data don't get more data. */
     if (s->byte_left == 0) {
         s->audio_data_available[0] = xlnx_dpdma_start_operation(s->dpdma, 4,
                                                                   true);