docs: Fix migration documentation for SDL_WINDOWEVENT_SIZE_CHANGED

Mark the event as removed as there is no true direct mapping in SDL3, and inform clients that they should handle the explicit SDL_EVENT_WINDOW_RESIZED and SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED events instead.

docs/README-migration.md

@@ -2241,11 +2241,11 @@ The following symbols have been renamed:
 * SDL_WINDOWEVENT_RESIZED => SDL_EVENT_WINDOW_RESIZED
 * SDL_WINDOWEVENT_RESTORED => SDL_EVENT_WINDOW_RESTORED
 * SDL_WINDOWEVENT_SHOWN => SDL_EVENT_WINDOW_SHOWN
-* SDL_WINDOWEVENT_SIZE_CHANGED => SDL_EVENT_WINDOW_SIZE_CHANGED
 * SDL_WINDOW_ALLOW_HIGHDPI => SDL_WINDOW_HIGH_PIXEL_DENSITY
 * SDL_WINDOW_INPUT_GRABBED => SDL_WINDOW_MOUSE_GRABBED
 
 The following symbols have been removed:
+* SDL_WINDOWEVENT_SIZE_CHANGED - handle the SDL_EVENT_WINDOW_RESIZED and SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED events instead
 * SDL_WINDOWEVENT_TAKE_FOCUS
 
 The following window operations are now considered to be asynchronous requests and should not be assumed to succeed unless