Add note on start transition opt out

Author: brophdawg11

docs/upgrading/v6.md

@@ -141,6 +141,8 @@ You don't need to update anything unless you are using `React.lazy` _inside_ of
 
 Using `React.lazy` inside of a component is incompatible with `React.useTransition` (or other code that makes promises inside of components). Move `React.lazy` to the module scope and stop making promises inside of components. This is not a limitation of React Router but rather incorrect usage of React.
 
+<docs-info>We added a flag to opt-out of `React.startTransition` in v7 so you can use that to upgrade to v7 without adopting React transition-enabled navigations if needed. See the [transition docs][transitions] for more information.</docs-info>
+
 ### v7_fetcherPersist
 
 <docs-warning>If you are not using a `<RouterProvider>` you can skip this</docs-warning>
@@ -377,3 +379,4 @@ Congratulations, you're now on v7!
 [react-flushsync]: https://react.dev/reference/react-dom/flushSync
 [response-json]: https://developer.mozilla.org/en-US/docs/Web/API/Response/json
 [data-util]: https://api.reactrouter.com/v7/functions/react-router.data.html
+[transitions]: ../explanation/react-transitions