On some versions of GHC, documentation is at, for example: LIB/share/doc/ In others, it is at: LIB/share/doc/x86_64-darwin-ghc7.6.3/ This change will pick up documentation first from the second location, then from the first.
Nixpkgs is a collection of packages for Nix package manager. Nixpkgs also includes NixOS linux distribution source code.