Update NavigatorIOS component doc

Summary:
Reference: #8203

Changes made:

- Added more to the intro section and updated the intro examples to ES6
- Added more details to prop explanations
- Added parameter descriptions for methods

**Test plan (required)**

Ran the website locally and checked: http://localhost:8079/react-native/docs/navigatorios.html

![component_navigatorios_2](https://cloud.githubusercontent.com/assets/691109/16315939/1501ba2a-3939-11e6-8ec0-54b43e03b323.png)
Closes #8334

Differential Revision: D3476066

Pulled By: JoelMarcey

fbshipit-source-id: 9fcefe3f9d59008d8c72683c57cb004d1f185f62

Author: Christine Abernathy