nixos/security.sudo: describe extraRules order

The order of sudoers entries is significant. The man page for sudoers(5)
notes:

  Where there are multiple matches, the last match is used (which is not
  necessarily the most specific match).

This module adds a rule for group "wheel" matching all commands. If you
wanted to add a more specific rule allowing members of the "wheel" group
to run command `foo` without a password, you'd need to use mkAfter to
ensure your rule comes after the more general rule.

  extraRules = lib.mkAfter [
    {
      groups = [ "wheel" ];
      commands = [
        {
          command = "${pkgs.foo}/bin/foo";
          options = [ "NOPASSWD" "SETENV" ];
        }
      ]
    }
  ];

Otherwise, when configuration options are merged, if the general rule
ends up after the specific rule, it will dictate the behavior even when
running the `foo` command.