Document the SDL audio channel mapping
diff --git a/include/SDL_audio.h b/include/SDL_audio.h
index a6acf55..f119c2b 100644
--- a/include/SDL_audio.h
+++ b/include/SDL_audio.h
@@ -164,6 +164,15 @@
/**
* 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
{
