nixos/acme: default to lets encrypt production URL instead of null, mention...

      };

      server = mkOption {

        type = types.nullOr types.str;

        inherit (defaultAndText "server" null) default defaultText;

        type = types.str;

        inherit (defaultAndText "server" "https://acme-v02.api.letsencrypt.org/directory") default defaultText;

        example = "https://acme-staging-v02.api.letsencrypt.org/directory";

        description = lib.mdDoc ''

          ACME Directory Resource URI. Defaults to Let's Encrypt's

          production endpoint,

          <https://acme-v02.api.letsencrypt.org/directory>, if unset.

          ACME Directory Resource URI.

          Defaults to Let's Encrypt's production endpoint.

          For testing Let's Encrypt's [staging endpoint](https://letsencrypt.org/docs/staging-environment/)

          should be used to avoid the rather tight rate limit on the production endpoint.

        '';

      };