The module Language.Haskell.Extension serves as a kind of central registry of all Haskell language extensions. It is shared between the various Haskell implementations.
Currently it's just a big list which is quite unhelpful. All new extensions are now required to come with documentation but we need someone to go through the existing list and copy documentation and convert it into haddock format. The best existing source of documentation for language extensions is the ghc user guide.
This is a nice standalone task that anyone with a bit of spare time could do. No knowledge of Cabal is required. It can even be done incrementally. Send you patches to the cabal-devel mailing list or attach them to this ticket.
