Docs: Updated iOS Component Guide
Docs: Updated iOS Component Guide

Summary:
Follow up on https://github.com/facebook/react-native/issues/14436

hramos:
>  MapViewIOS was removed a couple of versions ago. No one has touched this guide in a while, so unless someone volunteers to get it back up to date, we'll probably end up removing it from the docs.

I'm volunteering to get it back up to date!

- Fixed broken code examples
- Swapped out `pitchEnabled` for `zoomEnabled`, needs less explaining and it's easier to test on a simulator.
- Renamed RNTMap to RNTMapView for clarity.
- Renamed inconsistent `onChange`s to onRegionChange`.
- Swapped out 'vending' for 'exposing'.

I wasn't really sure about the last 3 points. I think it's more clear like this, but please review.

I'll add some review comments as well.
Closes https://github.com/facebook/react-native/pull/14991

Differential Revision: D5416319

Pulled By: hramos

fbshipit-source-id: bc70942a66acc5e3967b245af8271a1d2465ab60

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