add a comment to SDL_InitSubSystem explaining its refcounting behavior

include/SDL.h

@@ -130,6 +130,12 @@ extern DECLSPEC int SDLCALL SDL_Init(Uint32 flags);
 
 /**
  *  This function initializes specific SDL subsystems
+ *
+ *  Subsystem initialization is ref-counted, you must call
+ *  SDL_QuitSubSystem for each SDL_InitSubSystem to correctly
+ *  shutdown a subsystem manually (or call SDL_Quit to force shutdown).
+ *  If a subsystem is already loaded then this call will
+ *  increase the ref-count and return.
  */
 extern DECLSPEC int SDLCALL SDL_InitSubSystem(Uint32 flags);