hint user that rollout may take time

[r10s]

seen a similar hint for other apps, it is useful to give interested users a clue why they have a feature but others may not.

counterpart of https://github.com/deltachat/deltachat-ios/pull/2194

[github-actions[bot]]

To test the changes in this pull request, install this apk: 📦 app-preview.apk