Summary:
In the current docs, it's not quite clear, at the first sight, what the `lang` parameter passed to `onValueChange` is. This makes it obvious.
Closes https://github.com/facebook/react-native/pull/13617
Differential Revision: D4969592
Pulled By: javache
fbshipit-source-id: e94bedefebbe19d838d010f0c79d9a76743a4341