Add a sentence to the end of componentWillUnmount() (#894)

Adds a sentence explaining not to use setState in componentWillUnmount, see https://github.com/facebook/react/issues/12111

content/docs/reference-react-component.md

@@ -329,7 +329,7 @@ If your component implements the `getSnapshotBeforeUpdate()` lifecycle, the valu
 componentWillUnmount()
 ```
 
-`componentWillUnmount()` is invoked immediately before a component is unmounted and destroyed. Perform any necessary cleanup in this method, such as invalidating timers, canceling network requests, or cleaning up any subscriptions that were created in `componentDidMount()`.
+`componentWillUnmount()` is invoked immediately before a component is unmounted and destroyed. Perform any necessary cleanup in this method, such as invalidating timers, canceling network requests, or cleaning up any subscriptions that were created in `componentDidMount()`. You should not call `setState()` here because the component will never be re-rendered.
 
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