Add default fn param test and improve docs

Author: vitorbal

docs/rules/require-default-props.md

@@ -2,7 +2,8 @@
 
 This rule aims to ensure that any non-required `PropType` declaration of a component has a corresponding `defaultProps` value.
 
-One advantage of `defaultProps` over regular default logic in your code is that `defaultProps` are resolved by React before the `PropTypes` typechecking happens, so typechecking will also apply to your `defaultProps`.
+One advantage of `defaultProps` over custom default logic in your code is that `defaultProps` are resolved by React before the `PropTypes` typechecking happens, so typechecking will also apply to your `defaultProps`.
+The same also holds true for stateless functional components: default function parameters do not behave the same as `defaultProps` and thus using `defaultProps` is still preferred.
 
 To illustrate, consider the following example:
 

tests/lib/rules/require-default-props.js

@@ -1029,6 +1029,21 @@ ruleTester.run('require-default-props', rule, {
         line: 8,
         column: 3
       }]
+    },
+    {
+      code: [
+        'var Greetings = ({ foo = "foo" }) => {',
+        '  return <div>Hello {this.props.name}</div>;',
+        '}',
+        'Greetings.propTypes = {',
+        '  foo: React.PropTypes.string',
+        '};'
+      ].join('\n'),
+      errors: [{
+        message: 'propType "foo" is not required, but has no corresponding defaultProp declaration.',
+        line: 5,
+        column: 3
+      }]
     }
   ]
 });