stunnel: make servers accept more lenient

stunnel config's accept syntax is [host:]port -- this is required to e.g. listen on ipv6
where one would set :::port

nixos/modules/services/networking/stunnel.nix

@@ -16,8 +16,12 @@ let
   serverConfig = {
     options = {
       accept = mkOption {
-        type = types.int;
-        description = "On which port stunnel should listen for incoming TLS connections.";
+        type = types.either types.str types.int;
+        description = ''
+          On which [host:]port stunnel should listen for incoming TLS connections.
+          Note that unlike other softwares stunnel ipv6 address need no brackets,
+          so to listen on all IPv6 addresses on port 1234 one would use ':::1234'.
+        '';
       };
 
       connect = mkOption {