treewide: add defaultText (manual)

This enables generating module option documentation.

Add `defaultText` to options where the default value can't be
evaluated during metadata generation.

modules/nodeinfo.nix

@@ -8,6 +8,7 @@ let
       program = mkOption {
         readOnly = true;
         default = script;
+        defaultText = "(See source)";
       };
     };
   };