Sync SDL3 header -> wiki

SDL3/SDL_SetWindowShape.md

@@ -29,12 +29,14 @@ bool SDL_SetWindowShape(SDL_Window *window, SDL_Surface *shape);
 
 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](SDL_SetWindowShape)() again to update the window.
+[SDL_SetWindowShape](SDL_SetWindowShape)() again to update the window. This
+is an expensive operation, so should be done sparingly.
 
 The window must have been created with the
 [SDL_WINDOW_TRANSPARENT](SDL_WINDOW_TRANSPARENT) flag.