Update `React.PureComponent` reference.

Maybe I'm somehow weird, but I lived for some time with an idea that `React.Component` does implement `shouldComponentUpdate` with deep comparison. `React.PureComponent` is faster because it compares shallowly.

The changes I propose should avoid this "implicit" kind of way of thinking, since it would explicitly state that `React.Component` does not compare anything.

content/docs/reference-react.md

@@ -71,9 +71,9 @@ 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.
+`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. 
 
-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.
+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. 
 
 > Note
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