Docs for language flags

[Neil Mitchell]

Hi

> The documentation for language extensions appears in GHC's user manual in *three* places:
>
> (a) The flag summary http://www.haskell.org/ghc/docs/latest/html/users_guide/flag-reference.html#id352860
>
> (b) A section at the beginning of Chapter 8: http://www.haskell.org/ghc/docs/latest/html/users_guide/ghc-language-features.html#options-language
>
> (c) The individual sections that describe the extensions.

The documentation should also appear in:

http://hackage.haskell.org/packages/archive/Cabal/1.4.0.1/doc/html/Language-Haskell-Extension.html

It might be easier if the information was duplicated there, as then it
would be hoogle-able (although making the GHC manual hoogle-able is on
the Hoogle bug list
http://code.google.com/p/ndmitchell/issues/detail?id=84)

Thanks

Neil