nixos/*: md-"convert" empty descriptions

for some reason these are not picked up properly by nix-doc-munge, so
we'll do this instead.

nixos/modules/services/backup/bacula.nix

···
       };
 
       devices = mkOption {
-        description = "";
+        description = lib.mdDoc "";
         type = types.listOf types.str;
       };
 

nixos/modules/services/cluster/hadoop/default.nix

···
       type = types.package;
       default = pkgs.hadoop;
       defaultText = literalExpression "pkgs.hadoop";
-      description = "";
+      description = lib.mdDoc "";
     };
   };
 

nixos/modules/services/mail/postfix.nix

···
       privileged = mkOption {
         type = types.bool;
         example = true;
-        description = "";
+        description = lib.mdDoc "";
       };
 
       chroot = mkOption {

nixos/modules/services/networking/quicktun.nix

···
             type = types.int;
             default = 0;
             example = 1;
-            description = "";
+            description = lib.mdDoc "";
           };
 
           remoteAddress = mkOption {
             type = types.str;
             example = "tunnel.example.com";
-            description = "";
+            description = lib.mdDoc "";
           };
 
           localAddress = mkOption {
             type = types.str;
             example = "0.0.0.0";
-            description = "";
+            description = lib.mdDoc "";
           };
 
           localPort = mkOption {
             type = types.int;
             default = 2998;
-            description = "";
+            description = lib.mdDoc "";
           };
 
           remotePort = mkOption {
             type = types.int;
             default = 2998;
-            description = "";
+            description = lib.mdDoc "";
           };
 
           remoteFloat = mkOption {
             type = types.int;
             default = 0;
-            description = "";
+            description = lib.mdDoc "";
           };
 
           protocol = mkOption {
             type = types.str;
             default = "nacltai";
-            description = "";
+            description = lib.mdDoc "";
           };
 
           privateKey = mkOption {
             type = types.str;
-            description = "";
+            description = lib.mdDoc "";
           };
 
           publicKey = mkOption {
             type = types.str;
-            description = "";
+            description = lib.mdDoc "";
           };
 
           timeWindow = mkOption {
             type = types.int;
             default = 5;
-            description = "";
+            description = lib.mdDoc "";
           };
 
           upScript = mkOption {
             type = types.lines;
             default = "";
-            description = "";
+            description = lib.mdDoc "";
           };
         };
       });

nixos/modules/services/networking/xinetd.nix

···
           flags = mkOption {
             type = types.str;
             default = "";
-            description = "";
+            description = lib.mdDoc "";
           };
 
           unlisted = mkOption {

nixos/modules/services/torrent/transmission.nix

···
           options.incomplete-dir-enabled = mkOption {
             type = types.bool;
             default = true;
-            description = "";
+            description = lib.mdDoc "";
           };
           options.message-level = mkOption {
             type = types.ints.between 0 3;

nixos/modules/system/boot/loader/raspberrypi/raspberrypi.nix

···
       version = mkOption {
         default = 2;
         type = types.enum [ 0 1 2 3 4 ];
-        description = "";
+        description = lib.mdDoc "";
       };
 
       uboot = {