Polished allies03a

- added difficulties
- removed prisoner hack
- added a description in order to fix #7084 

PS: What about something like "As we don't want to engage (too many) soviet reinforcements,
I'll need your explicit orders to fire, Commander!" instead of "The area is crowded with civilians"?
Yes, both `Shroud.IsVisible` and `Shroud.IsTargetable` care about cloaking (and cloak detection which is missing here) via `IVisibilityModifier` so this redundant function should go away.