modules: add mkAliasOptionModuleMD

mkAliasOptionModule should not default to mdDoc descriptions because
that can break out-of-tree users of documentation infrastructure. add an
explicitly-MD variant for now, to be removed some time after the MD
transition is complete.

nixos/lib/make-options-doc/mergeJSON.py

@@ -306,14 +306,16 @@ if hasDocBookErrors:
     print("Explanation: The documentation contains descriptions, examples, or defaults written in DocBook. " +
         "NixOS is in the process of migrating from DocBook to Markdown, and " +
         "DocBook is disallowed for in-tree modules. To change your contribution to "+
-        "use Markdown, apply mdDoc and literalMD. For example:\n" +
+        "use Markdown, apply mdDoc and literalMD and use the *MD variants of option creation " +
+        "functions where they are available. For example:\n" +
         "\n" +
         "  example.foo = mkOption {\n" +
         "    description = lib.mdDoc ''your description'';\n" +
         "    defaultText = lib.literalMD ''your description of default'';\n" +
-        "  }\n" +
+        "  };\n" +
         "\n" +
-        "  example.enable = mkEnableOption (lib.mdDoc ''your thing'');",
+        "  example.enable = mkEnableOption (lib.mdDoc ''your thing'');\n" +
+        "  imports = [ (mkAliasOptionModuleMD [ \"example\" \"args\" ] [ \"example\" \"settings\" ]) ];",
         file = sys.stderr)
 
 if hasErrors: