removed line as functions can have state now too (#1783)

* removed line as functions can have state now too

* removed this.setState line from docs

content/docs/reference-glossary.md

@@ -122,7 +122,7 @@ class Welcome extends React.Component {
 
 A component needs `state` when some data associated with it changes over time. For example, a `Checkbox` component might need `isChecked` in its state, and a `NewsFeed` component might want to keep track of `fetchedPosts` in its state.
 
-The most important difference between `state` and `props` is that `props` are passed from a parent component, but `state` is managed by the component itself. A component cannot change its `props`, but it can change its `state`. To do so, it must call `this.setState()`. Only components defined as classes can have state.
+The most important difference between `state` and `props` is that `props` are passed from a parent component, but `state` is managed by the component itself. A component cannot change its `props`, but it can change its `state`.
 
 For each particular piece of changing data, there should be just one component that "owns" it in its state. Don't try to synchronize states of two different components. Instead, [lift it up](/docs/lifting-state-up.html) to their closest shared ancestor, and pass it down as props to both of them.