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"?
I see. Then the proper way should be 
`a.TraitsImplementing<IVisibilityModifier>().Any(t => !t.IsVisible(a, self.Owner))` instead.