Document the formState option for hydrateRoot

This adds the `formState` option to the `hydrateRoot` reference. The option carries the form state produced by a Server Function form submission so that `useActionState` with a `permalink` returns the submitted state during hydration instead of the initial state, and it must match the `formState` passed to the server renderer. The option shipped in React 19.0 but was never documented.

Co-Authored-By: Claude Code (kimi-k3[1m]) <noreply@anthropic.com>

Author: eps1lon

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

@@ -45,6 +45,7 @@ React will attach to the HTML that exists inside the `domNode`, and take over ma
   * **optional** `onUncaughtError`: Callback called when an error is thrown and not caught by an Error Boundary. Called with the `error` that was thrown and an `errorInfo` object containing the `componentStack`.
   * **optional** `onRecoverableError`: Callback called when React automatically recovers from errors. Called with the `error` React throws, and an `errorInfo` object containing the `componentStack`. Some recoverable errors may include the original error cause as `error.cause`.
   * **optional** `identifierPrefix`: A string prefix React uses for IDs generated by [`useId`.](/reference/react/useId) Useful to avoid conflicts when using multiple roots on the same page. Must be the same prefix as used on the server.
+  * **optional** `formState`: The form state from a form submission handled by a [Server Function](/reference/rsc/server-functions). If the page was rendered on the server in response to a submission of a form that uses [`useActionState`](/reference/react/useActionState) with a `permalink`, pass the resulting form state so that `useActionState` returns the submitted state instead of the `initialState`. Must be the same value as the `formState` passed to the [server renderer.](/reference/react-dom/server/renderToPipeableStream#parameters) This is typically passed through by your framework.
 
 
 #### Returns {/*returns*/}