docs: Note SDL_UpdateWindowSurfaceRects can update beyond specified areas.

Reference Issue #8825.

include/SDL3/SDL_video.h

@@ -1729,6 +1729,11 @@ extern DECLSPEC int SDLCALL SDL_UpdateWindowSurface(SDL_Window *window);
  *
  * This function is equivalent to the SDL 1.2 API SDL_UpdateRects().
  *
+ * Note that this function will update _at least_ the rectangles specified,
+ * but this is only intended as an optimization; in practice, this might
+ * update more of the screen (or all of the screen!), depending on what
+ * method SDL uses to send pixels to the system.
+ *
  * \param window the window to update
  * \param rects an array of SDL_Rect structures representing areas of the
  *              surface to copy, in pixels