Merge #12796: nixos docs: show references to packages

(version 2) A better implementation of #10039, after #12357.
This time I did more thorough checking.

See commit messages for details.

nixos/modules/programs/ssh.nix

@@ -36,6 +36,7 @@ in
 
       askPassword = mkOption {
         type = types.str;
+        default = "${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass";
         description = ''Program used by SSH to ask for passwords.'';
       };
 
@@ -226,7 +227,5 @@ in
 
     environment.variables.SSH_ASKPASS = optionalString config.services.xserver.enable askPassword;
 
-    programs.ssh.askPassword = mkDefault "${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass";
-
   };
 }

nixos/modules/programs/venus.nix

@@ -99,6 +99,7 @@ in
       };
 
       outputTheme = mkOption {
+        default = "${pkgs.venus}/themes/classic_fancy";
         type = types.path;
         description = ''
           Directory containing a config.ini file which is merged with this one.
@@ -168,7 +169,5 @@ in
         startAt = cfg.dates;
       };
 
-    services.venus.outputTheme = mkDefault "${pkgs.venus}/themes/classic_fancy";
-
   };
 }