import withNavigation removed from button component (react-navigation#352)

* import withNavigation removed from button component

import { withNavigation } from 'react-navigation';

(Suggested) to be removed from first code example to make it more clear to newcomers that it can be used as one approach to receive the navigation prop.

* Update connecting-navigation-prop.md

Author: Roeefl

docs/connecting-navigation-prop.md

@@ -6,10 +6,12 @@ sidebar_label: Access the navigation prop from any component
 
 [`withNavigation`](with-navigation.html) is a higher order component which passes the `navigation` prop into a wrapped Component. It's useful when you cannot pass the `navigation` prop into the component directly, or don't want to pass it in case of a deeply nested child.
 
+
+An ordinary component that is not a screen component will not receive the navigation prop by default, for example in this `MyBackButton` component:
+
 ```javascript
 import React from 'react';
 import { Button } from 'react-native';
-import { withNavigation } from 'react-navigation';
 
 export default class MyBackButton extends React.Component {
   render() {