Add instructions to temporarily disable UIScene migration

[vashworth]

What information needs to be added?

To temporarily disable UIScene, you can add a _ in front of "Application Scene Manifest" in your Info.plist

https://docs.flutter.dev/release/breaking-changes/uiscenedelegate#migrate-info-plist

When you're ready to re-enable, remove the _

Where should this new content appear?

https://docs.flutter.dev/release/breaking-changes/uiscenedelegate

I would like to fix this problem.

• I will try and fix this problem on docs.flutter.dev.