haddock-library - small changes to allow building on ghc 7.4

The one-line use of LambdaCase was preventing the library from building on ghc 7.4. I try to support ghc 7.4 with pandoc, so I'd like haddock-library to build there too.  This pull request replaces the use of LambdaCase with a regular case statement, and fixes a couple of comments that were generating haddock warnings.