Fix Haddock generation

Some Haddock comments were written improperly so the documentation was not being generated. See this [Hackage build log](http://hackage.haskell.org/package/persistent-redis-0.0.3/reports/1/log) for details.

See [Haddock manual chapter 2](http://www.haskell.org/haddock/doc/html/ch03s02.html) which explains how to comment such entities.