docs: migration guide note that SDL_LoadWAV has a different return type.

docs/README-migration.md

@@ -172,6 +172,8 @@ SDL_FreeWAV has been removed and calls can be replaced with SDL_free.
 
 SDL_LoadWAV() is a proper function now and no longer a macro (but offers the same functionality otherwise).
 
+SDL_LoadWAV_RW() and SDL_LoadWAV() return an int now: zero on success, -1 on error, like most of SDL. They no longer return a pointer to an SDL_AudioSpec.
+
 SDL_AudioCVT interface has been removed, the SDL_AudioStream interface (for audio supplied in pieces) or the new SDL_ConvertAudioSamples() function (for converting a complete audio buffer in one call) can be used instead.
 
 Code that used to look like this: