Improved the documentation for SDL_SetWindowShape()

Fixes https://github.com/libsdl-org/SDL/issues/11199

include/SDL3/SDL_video.h

@@ -2459,12 +2459,12 @@ extern SDL_DECLSPEC bool SDLCALL SDL_SetWindowHitTest(SDL_Window *window, SDL_Hi
  *
  * This sets the alpha channel of a transparent window and any fully
  * transparent areas are also transparent to mouse clicks. If you are using
- * something besides the SDL render API, then you are responsible for setting
- * the alpha channel of the window yourself.
+ * something besides the SDL render API, then you are responsible for drawing
+ * the alpha channel of the window to match the shape alpha channel to get consistent cross-platform results. 
  *
  * The shape is copied inside this function, so you can free it afterwards. If
  * your shape surface changes, you should call SDL_SetWindowShape() again to
- * update the window.
+ * update the window. This is an expensive operation, so should be done sparingly.
  *
  * The window must have been created with the SDL_WINDOW_TRANSPARENT flag.
  *