Tweaked message about what goes into state

content/docs/state-and-lifecycle.md

@@ -251,7 +251,7 @@ Note how we save the timer ID right on `this`.
 
 While `this.props` is set up by React itself and `this.state` has a special meaning, you are free to add additional fields to the class manually if you need to store something that is not used for the visual output.
 
-If you don't use something in `render()`, it shouldn't be in the state.
+State is generally used to store values that are referenced by `render()` (for visual output) or by `getDerivedStateFromProps()` (to compare next and previous props values).
 
 We will tear down the timer in the `componentWillUnmount()` lifecycle hook: