docs: add warning to not call the resolver more than once

[jlnrrg]

Warning to not resolve more than once

The isResolved guard clause prevents redirect to be called more than once.

  /// Redirect to a different path
  void redirect(String path) {
    if (isResolved) return;
    _completer.complete(GuardResult.redirect(path));
  }

It is still possible to create a RouteGuard with mulitple calls to .redirect which will not work as expected.
This might be a normal pattern for this kind of implementation, but people unknown with the resolver pattern micht benefit from a warning in the documentation.

Example Guard

class TestGuard extends RouteGuard {
  @override
  FutureOr<void> onNavigation(NavigationResolver resolver, BuildContext context, GoRouterState state) async {
    resolver.next();
    resolver.redirect('/');
    // Do something here
    await Future.delayed(const Duration(seconds: 2));
    resolver.redirect('login');
  }
}

Btw. I am not arguing that is a good idea to call it twice or thrice.