Updated the migration guide to note that you can check the return value of SDL_AddEventWatch()

docs/README-migration.md

@@ -314,6 +314,8 @@ The gamepad event structures caxis, cbutton, cdevice, ctouchpad, and csensor hav
 
 SDL_QUERY, SDL_IGNORE, SDL_ENABLE, and SDL_DISABLE have been removed. You can use the functions SDL_SetEventEnabled() and SDL_EventEnabled() to set and query event processing state.
 
+SDL_AddEventWatch() now returns -1 if it fails because it ran out of memory and couldn't add the event watch callback.
+
 The following symbols have been renamed:
 * SDL_APP_DIDENTERBACKGROUND => SDL_EVENT_DID_ENTER_BACKGROUND
 * SDL_APP_DIDENTERFOREGROUND => SDL_EVENT_DID_ENTER_FOREGROUND