ao_sndio: add notice about padding channels

(I won't do this, but someone else seeing this might.)

audio/out/ao_sndio.c

@@ -98,7 +98,9 @@ static const struct mp_chmap sndio_layouts[MP_NUM_CHANNELS + 1] = {
     {0},                                        // 6.1
     MP_CHMAP8(FL, FR, BL, BR, FC, LFE, SL, SR), // 7.1
     /* above is the fixed channel assignment for sndio, since we need to fill
-       all channels and cannot insert silence, not all layouts are supported. */
+       all channels and cannot insert silence, not all layouts are supported.
+       NOTE: MP_SPEAKER_ID_NA0-7 could be used to add padding channels. Keep
+             in mind that they don't actually contain silence. */
 };
 
 /*