Clean up examples in prop-types docs

These examples use `createReactClass` a lot, which is legacy at this
point. I'm modifying examples to primarily use function components
because those are the ones people are using most often today. Other ways
of defining a component are just here to show that this rule recognizes
them, too.

docs/rules/prop-types.md

@@ -9,29 +9,43 @@ It can warn other developers if they make a mistake while reusing the component
 The following patterns are considered warnings:
 
 ```jsx
-var Hello = createReactClass({
-  render: function() {
-    return <div>Hello {this.props.name}</div>;
-  }
-});
+function Hello({ name }) {
+  return <div>Hello {name}</div>;
+  // 'name' is missing in props validation
+}
 
 var Hello = createReactClass({
   propTypes: {
     firstname: PropTypes.string.isRequired
   },
   render: function() {
-    return <div>Hello {this.props.firstname} {this.props.lastname}</div>; // lastname type is not defined in propTypes
+    return <div>Hello {this.props.firstname} {this.props.lastname}</div>;
+    // 'lastname' type is missing in props validation
   }
 });
 
-function Hello({ name }) {
-  return <div>Hello {name}</div>;
+// Or in ES6
+class Hello extends React.Component {
+  render() {
+    return <div>Hello {this.props.firstname} {this.props.lastname}</div>;
+    // 'lastname' type is missing in props validation
+  }
+}
+Hello.propTypes = {
+  firstname: PropTypes.string.isRequired
 }
 ```
 
 Examples of correct usage without warnings:
 
 ```jsx
+function Hello({ name }) {
+  return <div>Hello {name}</div>;
+}
+Hello.propTypes = {
+  name: PropTypes.string.isRequired
+}
+
 var Hello = createReactClass({
   propTypes: {
     name: PropTypes.string.isRequired,
@@ -65,35 +79,15 @@ class HelloEs6WithPublicClassField extends React.Component {
 The following patterns are **not** considered warnings:
 
 ```jsx
-var Hello = createReactClass({
-  render: function() {
-    return <div>Hello World</div>;
-  }
-});
-
-var Hello = createReactClass({
-  propTypes: {
-    name: PropTypes.string.isRequired
-  },
-  render: function() {
-    return <div>Hello {this.props.name}</div>;
-  }
-});
+function Hello() {
+  return <div>Hello World</div>;
+}
 
 // Referencing an external object disable the rule for the component
-var Hello = createReactClass({
-  propTypes: myPropTypes,
-  render: function() {
-    return <div>Hello {this.props.name}</div>;
-  }
-});
-
 function Hello({ name }) {
   return <div>Hello {name}</div>;
 }
-Hello.propTypes = {
-  name: PropTypes.string.isRequired,
-};
+Hello.propTypes = myPropTypes;
 ```
 
 ## Rule Options
@@ -135,6 +129,6 @@ For this rule to work we need to detect React components, this could be very har
 
 For now we should detect components created with:
 
+* a function that return JSX or the result of a `React.createElement` call.
 * `createReactClass()`
 * an ES6 class that inherit from `React.Component` or `Component`
-* a stateless function that return JSX or the result of a `React.createElement` call.