I for one would be overjoyed if someone could take this page on.
It's the second oldest issue on SpongeDocs and seriously needs some love. However, the one best qualified for it, the esteemed @zml, is deeply involved in the mad race of development leading to Beta, and may not have much time for documentation.
So please accept out apologies, but no-one well enough versed in the Sponge permissions API has yet stepped forward to take on this challenge. The best advice I can offer at this point is to ask for specific help on #spongedev IRC, or in the Plugin development section of these forums. If you receive enough wisdom, maybe you can help write the docs