Summary:**Problem**: As seen in https://github.com/facebook/react-native/issues/5820, many devs are confused by the fact that the offline bundle is not generated when running against the simulator, even when running in the "Release" configuration which is supposed to mimic "production" scenarios.
This pull request is a small change that fixes https://github.com/facebook/react-native/issues/5820 by updating the `react-native-xcode.sh` shell script to still generate the ofline bundle during Release configuration. It also updates `AppDelegate.m` to better document this behaviour in the comments so as to avoid any surprises.
**Test plan**: This is a simple change, the two tests done were
1. In a new React Native project, verify that an offline build is not generated when running against the simulator in "Debug" configuration as per normal.
2. Change to a "Release" configuration via Product > Scheme > Edit Scheme in XCode, then verify that the project runs with the offline build generated.
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