nixos/*: convert internal option descriptions to MD

we'll have to do it eventually, may as well be now.

nixos/modules/misc/passthru.nix

@@ -7,7 +7,7 @@
   options = {
     passthru = lib.mkOption {
       visible = false;
-      description = ''
+      description = lib.mdDoc ''
         This attribute set will be exported as a system attribute.
         You can put whatever you want here.
       '';