add/remove some comments

src/audio.h

@@ -16,7 +16,6 @@
 #define DEFAULT_SFX_VOLUME 100
 
 #define LOOPFADEOUT 50
-// XXX: THIS IS SPARTA. but this needs to be a property set in volumes.conf or wherever
 
 typedef struct CurrentBGM {
 	char *name;

src/audio_mixer.c

@@ -360,6 +360,7 @@ bool audio_backend_sound_loop(void *impl, AudioBackendSoundGroup group) {
 	return true;
 }
 
+// XXX: This custom fading effect circumvents https://bugzilla.libsdl.org/show_bug.cgi?id=2904
 typedef struct CustomFadeout {
 	int duration; // in samples
 	int counter;