Understanding persistence modes for ConcUnlift

[torgeirsh]

In the situations where I've used ConcUnlift, it doesn't seem to matter which mode I use, so I don't know which one is more correct in edge cases. The documentation mentions which mode to use in a couple of cases, but it would be helpful to see concrete examples of how things can go wrong to get a better understanding.

[arybczak]

This would be a good augmentation of the documentation, I created #318 so I don't forget to do that.