Summary:
The example of `BackAndroid` caused a bit of confusion for beginners (see #8822), so I thought we should clarify the functionality a bit. I added a comment indicating that a user would have to implement `this.onMainScreen` and `this.goBack` on their own, this was the original problem.
Closes https://github.com/facebook/react-native/pull/8829
Differential Revision: D3660332
fbshipit-source-id: d84a29586c6a1a439f386e6b88220d7b0a275129