Fix up Haddock comments

Documenting GADT constructors this way is not supported in Haddock. See [this ticket](http://trac.haskell.org/haddock/ticket/43). The best one can do is to use GADT records and comment the fields.

Due to this, Hackage documentation wasn't being built. See [this build log](http://hackage.haskell.org/package/FormalGrammars-0.0.0.2/reports/1/log) for details.

I worked around the issue by documenting the constructors in the data declaration comment instead.

It now looks like this:
![GADT constructor documentation](http://fuuzetsu.co.uk/images/1388921814.png)