Clarify purpose of state property in ES6 classes (#7109)

(cherry picked from commit 60760eb915)

docs/docs/05-reusable-components.md

@@ -213,7 +213,7 @@ class HelloMessage extends React.Component {
 ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 ```
 
-The API is similar to `React.createClass` with the exception of `getInitialState`. Instead of providing a separate `getInitialState` method, you set up your own `state` property in the constructor.
+The API is similar to `React.createClass` with the exception of `getInitialState`. Instead of providing a separate `getInitialState` method, you set up your own `state` property in the constructor. Just like the return value of `getInitialState`, the value you assign to `this.state` will be used as the initial state for your component.
 
 Another difference is that `propTypes` and `defaultProps` are defined as properties on the constructor instead of in the class body.