Add note about root.render microtask scheduling (#7717)

Co-authored-by: Sebastian "Sebbie" Silbermann <sebastian.silbermann@vercel.com>

Author: jukben

src/content/reference/react-dom/client/createRoot.md

@@ -90,6 +90,15 @@ React will display `<App />` in the `root`, and take over managing the DOM insid
 
 * If you call `render` on the same root more than once, React will update the DOM as necessary to reflect the latest JSX you passed. React will decide which parts of the DOM can be reused and which need to be recreated by ["matching it up"](/learn/preserving-and-resetting-state) with the previously rendered tree. Calling `render` on the same root again is similar to calling the [`set` function](/reference/react/useState#setstate) on the root component: React avoids unnecessary DOM updates.
 
+* Although rendering is synchronous once it starts, `root.render(...)` is not. This means code after `root.render()` may run before any effects (`useLayoutEffect`, `useEffect`) of that specific render are fired. This is usually fine and rarely needs adjustment. In rare cases where effect timing matters, you can wrap `root.render(...)` in [`flushSync`](https://react.dev/reference/react-dom/client/flushSync) to ensure the initial render runs fully synchronously.
+  
+  ```js
+  const root = createRoot(document.getElementById('root'));
+  root.render(<App />);
+  // 🚩 The HTML will not include the rendered <App /> yet:
+  console.log(document.body.innerHTML);
+  ```
+
 ---
 
 ### `root.unmount()` {/*root-unmount*/}