There is no official spoiler policy. And honestly, the bit about not spoiling the Dragonslayer was started by the players, not the administration.
It's pretty much a common sense thing. If you want to be spoiled, there's a wiki that's full of information about DoomRL and will spoil the bejeezus out of you. If you're letting out a huge drop of information here, just let people know via spoiler tags, or with a big "SPOILERS" sign. They can chose to spoil themselves or not, either way.