lib: add mdDoc back in to provide proper warning

lib/default.nix

···
       scrubOptionValue literalExpression literalExample
       showOption showOptionWithDefLocs showFiles
       unknownModule mkOption mkPackageOption mkPackageOptionMD
-      literalMD;
+      mdDoc literalMD;
     inherit (self.types) isType setType defaultTypeMerge defaultFunctor
       isOptionType mkOptionType;
     inherit (self.asserts)