manual-combined.xml:7474: element link: validity error : IDREF attribute linkend references an unknown ID "users.users._name_.extraGroups"
  7470
  7471  # /var/lib/acme/.challenges must be writable by the ACME user
  7472  # and readable by the Nginx user. The easiest way to achieve
  7473  # this is to add the Nginx user to the ACME group.
  7474  <link linkend="users.users._name_.extraGroups">users.users.nginx.extraGroups</link> = [ "acme" ];
  7475
  7476  services.nginx = {

manual-combined.xml:7492: element link: validity error : IDREF attribute linkend references an unknown ID "users.users._name_.extraGroups"
  7488      };
  7489    };
  7490  }
  7491  # Alternative config for Apache
  7492  <link linkend="users.users._name_.extraGroups">users.users.wwwrun.extraGroups</link> = [ "acme" ];
  7493  services.httpd = {
  7494    <link linkend="opt-services.httpd.enable">enable = true;</link>

manual-combined.xml fails to validate
builder for '/nix/store/66wp67l65wbnkdwdl0lv9l41b37ynfh4-nixos-manual-combined.drv' failed with exit code 3