Fix concurrent mode ffi, add more warnings about usage

Author: Madeline Trotter

src/React/Basic/DOM/Concurrent.js

@@ -1,6 +1,7 @@
 "use strict";
 
 const ReactDOM = require("react-dom");
+const createRoot = ReactDOM.createRoot || ReactDOM.unstable_createRoot;
 
 exports.renderConcurrentMode = (jsx) => (element) => () =>
-  ReactDOM.createRoot(element).render(jsx);
+  createRoot(element).render(jsx);

src/React/Basic/DOM/Concurrent.purs

@@ -1,3 +1,5 @@
+-- Warning: The concurrent-mode API is experimental and not yet intended for
+-- use in production applications. See the React docs for more info.
 module React.Basic.DOM.Concurrent where
 
 import Prelude
@@ -6,5 +8,5 @@ import React.Basic (JSX)
 import Web.DOM (Element)
 
 -- Alternative to `render` which enables React's concurrent mode.
--- This is an experimental feature. See the React docs for more info.
+-- This is an unstable and experimental feature. See the React docs for more info.
 foreign import renderConcurrentMode :: JSX -> Element -> Effect Unit