[Docs] Update docs around SFC instances in React 16

docs/api/ShallowWrapper/instance.md

@@ -2,20 +2,62 @@
 
 Gets the instance of the component being rendered as the root node passed into `shallow()`.
 
-NOTE: can only be called on a wrapper instance that is also the root instance.
+NOTE: can only be called on a wrapper instance that is also the root instance. With React `16.x`, `instance()` returns `null` for stateless React component/stateless functional components. See example:
 
+#### Returns (React 16.x)
 
+- `ReactComponent`: The stateful React component instance.
+- `null`: If stateless React component was wrapped.
 
-#### Returns
+#### Returns (React 15.x)
 
-`ReactComponent`: The component instance.
+- `ReactComponent`: The component instance.
 
+#### Example
+
+#### Preconditions
+<!-- eslint react/prop-types: 0, react/prefer-stateless-function: 0 -->
+```jsx
+function Stateless() {
+  return <div>Stateless</div>;
+}
+
+class Stateful extends React.Component {
+  render() {
+    return <div>Stateful</div>;
+  }
+}
+```
+#### React 16.x
+```jsx
+test('shallow wrapper instance should not be null', () => {
+  const wrapper = shallow(<Stateless />);
+  const instance = wrapper.instance();
 
+  expect(instance).not.to.be.instanceOf(Stateless);
+});
 
-#### Example
+test('shallow wrapper instance should not be null', () => {
+  const wrapper = shallow(<Stateful />);
+  const instance = wrapper.instance();
 
+  expect(instance).to.be.instanceOf(Stateful);
+});
+```
+#### React 15.x
 ```jsx
-const wrapper = shallow(<MyComponent />);
-const inst = wrapper.instance();
-expect(inst).to.be.instanceOf(MyComponent);
+test('shallow wrapper instance should not be null', () => {
+  const wrapper = shallow(<Stateless />);
+  const instance = wrapper.instance();
+
+  expect(instance).to.be.instanceOf(Stateless);
+});
+
+test('shallow wrapper instance should not be null', () => {
+  const wrapper = shallow(<Stateful />);
+  const instance = wrapper.instance();
+
+  expect(instance).to.be.instanceOf(Stateful);
+});
 ```
+

docs/api/ShallowWrapper/props.md

@@ -5,13 +5,13 @@ called on a wrapper of a single node.
 
 NOTE: When called on a shallow wrapper, `.props()` will return values for
 props on the root node that the component *renders*, not the component itself.
-To return the props for the entire React component, use `wrapper.instance().props`.
-See [`.instance() => ReactComponent`](instance.md)
 
+To return the props for the entire React component, use `wrapper.instance().props`. This is valid for stateful or stateless components in React `15.*`. But, `wrapper.instance()` will return `null` for stateless React component in React `16.*`, so `wrapper.instance().props` will cause an error in this case. See [`.instance() => ReactComponent`](instance.md)
 
-#### Example
 
+#### Example
 ```jsx
+
 import PropTypes from 'prop-types';
 
 function MyComponent(props) {
@@ -34,8 +34,11 @@ expect(wrapper.props().includedProp).to.equal('Success!');
 console.log(wrapper.props());
 // {children: "Hello", className: "foo bar", includedProp="Success!"}
 
-console.log(wrapper.instance().props);
+console.log(wrapper.instance().props); // React 15.x - working as expected
 // {children: "Hello", className: "foo bar", includedProp:"Success!", excludedProp: "I'm not included"}
+
+console.log(wrapper.instance().props);
+// React 16.* - Uncaught TypeError: Cannot read property 'props' of null
 ```