Really the only UN war law that most everyone takes seriously and is clearly spelled out is the one concerning mercenary forces. Basically if you are caught acting as a mercenary by the other side, you void having any of the rights guranteed by the Geneva Convention. Which means the people who have captured you can torture you, black mail you, starve you, brainwash you, or even execute you, and its perfectly legal.
Thats why you never really hear about groups officially referring to themselves as mercenaries after WW2. (today there are still mercenaries, but they call themselves "private contractors" which lets them fight as a mercenary and still have human rights benefits through a vocabulary loophole)