Removed createBlockingRoot API docs and added a warning note

Author: Brian Vaughn

content/docs/concurrent-mode-adoption.md

@@ -21,6 +21,10 @@ next: concurrent-mode-reference.html
 >
 >This documentation is aimed at early adopters and people who are curious. **If you're new to React, don't worry about these features** -- you don't need to learn them right now.
 
+>Caution:
+>
+>All references below to "blocking mode" and `createBlockingRoot` are outdated and should be ignored.
+
 </div>
 
 - [Installation](#installation)

content/docs/concurrent-mode-reference.md

@@ -28,7 +28,6 @@ This page is an API reference for the React [Concurrent Mode](/docs/concurrent-m
 
 - [Enabling Concurrent Mode](#concurrent-mode)
     - [`createRoot`](#createroot)
-    - [`createBlockingRoot`](#createblockingroot)
 - [Suspense](#suspense)
     - [`Suspense`](#suspensecomponent)
     - [`SuspenseList`](#suspenselist)
@@ -47,18 +46,6 @@ Replaces `ReactDOM.render(<App />, rootNode)` and enables Concurrent Mode.
 
 For more information on Concurrent Mode, check out the [Concurrent Mode documentation.](/docs/concurrent-mode-intro.html)
 
-### `createBlockingRoot` {#createblockingroot}
-
-```js
-ReactDOM.createBlockingRoot(rootNode).render(<App />)
-```
-
-Replaces `ReactDOM.render(<App />, rootNode)` and enables [Blocking Mode](/docs/concurrent-mode-adoption.html#migration-step-blocking-mode).
-
-Opting into Concurrent Mode introduces semantic changes to how React works. This means that you can't use Concurrent Mode in just a few components. Because of this, some apps may not be able to migrate directly to Concurrent Mode. 
-
-Blocking Mode only contains a small subset of Concurrent Mode features and is intended as an intermediary migration step for apps that are unable to migrate directly.
-
 ## Suspense API {#suspense}
 
 ### `Suspense` {#suspensecomponent}