Summary:
Explain that, unlike other APIs, geolocation follows the browser spec and is exposed through `navigator.geolocation` rather than as an `react-native` export.
This can be inferred from the example code but isn't otherwise stated in the docs. See also https://github.com/facebook/react-native/pull/9793
Closes https://github.com/facebook/react-native/pull/9810
Differential Revision: D3841341
Pulled By: javache
fbshipit-source-id: b1423e8bf7fb78c788f5cdc5630a4a948b8178c6