include: Clarify lifetime of RWops in Mix_LoadMUS* calls with freesrc!=0.

Fixes #683.

(cherry picked from commit bd3185b)

Author: icculus

include/SDL_mixer.h

@@ -665,9 +665,11 @@ extern DECLSPEC Mix_Music * SDLCALL Mix_LoadMUS(const char *file);
  * fly. Also, crucially, there are as many channels for chunks as the app can
  * allocate, but SDL_mixer only offers a single "music" channel.
  *
- * If `freesrc` is non-zero, the RWops will be closed before returning,
- * whether this function succeeds or not. SDL_mixer reads everything it needs
- * from the RWops during this call in any case.
+ * If `freesrc` is non-zero, the RWops will be closed when SDL_mixer is done
+ * with it, which will be before this function call returns if there is an
+ * error, or perhaps much later if the music is streaming for some time. The
+ * app should not attempt to use the RWops again, as it may become invalid
+ * without warning.
  *
  * As a convenience, there is a function to read files from disk without
  * having to deal with SDL_RWops: `Mix_LoadMUS("filename.mp3")` will manage
@@ -725,9 +727,11 @@ extern DECLSPEC Mix_Music * SDLCALL Mix_LoadMUS_RW(SDL_RWops *src, int freesrc);
  * - `MUS_OPUS` (Opus files)
  * - `MUS_WAVPACK` (WavPack files)
  *
- * If `freesrc` is non-zero, the RWops will be closed before returning,
- * whether this function succeeds or not. SDL_mixer reads everything it needs
- * from the RWops during this call in any case.
+ * If `freesrc` is non-zero, the RWops will be closed when SDL_mixer is done
+ * with it, which will be before this function call returns if there is an
+ * error, or perhaps much later if the music is streaming for some time. The
+ * app should not attempt to use the RWops again, as it may become invalid
+ * without warning.
  *
  * As a convenience, there is a function to read files from disk without
  * having to deal with SDL_RWops: `Mix_LoadMUS("filename.mp3")` will manage