Document the SDL audio channel mapping

include/SDL_audio.h

@@ -164,6 +164,15 @@ typedef void (SDLCALL * SDL_AudioCallback) (void *userdata, Uint8 * stream,
 
 /**
  *  The calculated values in this structure are calculated by SDL_OpenAudio().
+ *
+ *  For multi-channel audio, the default SDL channel mapping is:
+ *  2:  FL FR                       (stereo)
+ *  3:  FL FR LFE                   (2.1 surround)
+ *  4:  FL FR BL BR                 (quad)
+ *  5:  FL FR FC BL BR              (quad + center)
+ *  6:  FL FR FC LFE SL SR          (5.1 surround - last two can also be BL BR)
+ *  7:  FL FR FC LFE BC SL SR       (6.1 surround)
+ *  8:  FL FR FC LFE BL BR SL SR    (7.1 surround)
  */
 typedef struct SDL_AudioSpec
 {