Update `React.PureComponent` reference.

Rewording the `React.PureComponent` reference to make it more clear, that `React.Component` doesn't implement `.shouldComponentUpdate` at all.

content/docs/reference-react.md

@@ -70,8 +70,7 @@ See the [React.Component API Reference](/docs/react-component.html) for a list o
 * * *
 
 ### `React.PureComponent`
-
-`React.PureComponent` is exactly like [`React.Component`](#reactcomponent), but implements [`shouldComponentUpdate()`](/docs/react-component.html#shouldcomponentupdate) with a shallow prop and state comparison, which [`React.Component`](#reactcomponent) is missing completely. 
+`React.PureComponent` is similar to [`React.Component`](#reactcomponent). The difference between them is that [`React.Component`](#reactcomponent) doesn't implement [`shouldComponentUpdate()`](/docs/react-component.html#shouldcomponentupdate), but `React.PureComponent` implements it with a shallow prop and state comparison. 
 
 If your React component's `render()` function renders the same result given the same props and state, you can use `React.PureComponent` for a performance boost in some cases.