nixos: Provide a defaultText for type = package

We don't want to build all those things along with the manual, so that's
what the defaultText attribute is for.

Unfortunately a few of them were missing, so let's add them.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/services/computing/slurm/slurm.nix

@@ -37,6 +37,7 @@ in
       package = mkOption {
         type = types.package;
         default = pkgs.slurm-llnl;
+        defaultText = "pkgs.slurm-llnl";
         example = literalExample "pkgs.slurm-llnl-full";
         description = ''
           The packge to use for slurm binaries.