[Documentation] Impreove the react-component section of doc (#9349)

* Impreove react-component of doc 

[#9304](https://github.com/facebook/react/issues/9304)

* update description

* add missing space

docs/reference-react-component.md

@@ -136,7 +136,7 @@ If you "fork" props by using them for state, you might also want to implement [`
 componentWillMount()
 ```
 
-`componentWillMount()` is invoked immediately before mounting occurs. It is called before `render()`, therefore setting state in this method will not trigger a re-rendering. Avoid introducing any side-effects or subscriptions in this method.
+`componentWillMount()` is invoked immediately before mounting occurs. It is called before `render()`, therefore setting state synchronously in this method will not trigger a re-rendering. Avoid introducing any side-effects or subscriptions in this method.
 
 This is the only lifecycle hook called on server rendering. Generally, we recommend using the `constructor()` instead.