Sync SDL3 wiki -> header

[ci skip]

include/SDL3/SDL_log.h

@@ -487,7 +487,8 @@ typedef void (SDLCALL *SDL_LogOutputFunction)(void *userdata, int category, SDL_
 /**
  * Get the default log output function.
  *
- * \returns the default log output callback.
+ * \returns the default log output callback. It should be called with NULL for
+ *          the userdata argument.
  *
  * \threadsafety It is safe to call this function from any thread.
  *