Update documentation in default.nix

default.nix

-# sources.nix contains the nixpkgs checkout that is going
-# to be used on all hosts.
+# This file defines which commands are available
+# To run a command:
+# $ nix-build -A isoImage
 let
+  # sources.nix contains the nixpkgs checkout that is going
+  # to be used on all hosts.
   sources = import ./nix/sources.nix;
   pkgs = import sources.nixpkgs {
     system = "x86_64-linux";
@@ -12,14 +15,12 @@ let
   };
 in {
   inherit sources pkgs;
-  inherit (pkgs) lib; # inherit lib from pkgs
+  inherit (pkgs) lib; # the syntax means: "inherit lib from pkgs"
   
-  # Pass the pkgs from sources.nix to each module under lib/
+  # The following commands are available:
   kexec_tarball = import ./lib/kexec-tarball.nix { inherit pkgs; };
   isoImage = import ./lib/iso-image.nix { inherit pkgs; };
   deploy = import ./lib/deploy.nix { inherit pkgs; };
-  
-  # not needed right now
-  #unlock = import ./lib/unlock-fde.nix { inherit pkgs; };
+  #unlock = import ./lib/unlock-fde.nix { inherit pkgs; }; # not needed right now
 }
   // (import ./lib/hosts.nix { inherit pkgs; })