nixos/gitolite: add 'description' module option

This option allows for the customization of the description of the
created gitolite user.

An example of this being useful is for the integration of gitolite with
cgit, which itself uses the gitolite user's description as the author of
the git repo displayed in its generated site.

nixos/modules/services/misc/gitolite.nix

@@ -101,6 +101,14 @@ in
         '';
       };
 
+      description = mkOption {
+        type = types.str;
+        default = "Gitolite user";
+        description = lib.mdDoc ''
+          Gitolite user account's description.
+        '';
+      };
+
       group = mkOption {
         type = types.str;
         default = "gitolite";
@@ -145,7 +153,7 @@ in
     '';
 
     users.users.${cfg.user} = {
-      description     = "Gitolite user";
+      description     = cfg.description;
       home            = cfg.dataDir;
       uid             = config.ids.uids.gitolite;
       group           = cfg.group;