lib: turn *MD functions into aliases

with docbook gone and MD the default these aren't needed any more. we
can't remove them yet because there's thousands of uses, but maybe some
day we can.

lib/modules.nix

@@ -1146,14 +1146,11 @@ let
     use = id;
   };
 
-  /* Transitional version of mkAliasOptionModule that uses MD docs. */
-  mkAliasOptionModuleMD = from: to: doRename {
-    inherit from to;
-    visible = true;
-    warn = false;
-    use = id;
-    markdown = true;
-  };
+  /* Transitional version of mkAliasOptionModule that uses MD docs.
+
+     This function is no longer necessary and merely an alias of `mkAliasOptionModule`.
+  */
+  mkAliasOptionModuleMD = mkAliasOptionModule;
 
   /* mkDerivedConfig : Option a -> (a -> Definition b) -> Definition b
 
@@ -1175,7 +1172,7 @@ let
       (opt.highestPrio or defaultOverridePriority)
       (f opt.value);
 
-  doRename = { from, to, visible, warn, use, withPriority ? true, markdown ? false }:
+  doRename = { from, to, visible, warn, use, withPriority ? true }:
     { config, options, ... }:
     let
       fromOpt = getAttrFromPath from options;
@@ -1186,9 +1183,7 @@ let
     {
       options = setAttrByPath from (mkOption {
         inherit visible;
-        description = if markdown
-          then lib.mdDoc "Alias of {option}`${showOption to}`."
-          else "Alias of <option>${showOption to}</option>.";
+        description = "Alias of {option}`${showOption to}`.";
         apply = x: use (toOf config);
       } // optionalAttrs (toType != null) {
         type = toType;

lib/options.nix

@@ -100,10 +100,7 @@ rec {
     name: mkOption {
     default = false;
     example = true;
-    description =
-      if name ? _type && name._type == "mdDoc"
-      then lib.mdDoc "Whether to enable ${name.text}."
-      else "Whether to enable ${name}.";
+    description = "Whether to enable ${name}.";
     type = lib.types.bool;
   };
 
@@ -185,10 +182,10 @@ rec {
           (if isList example then "pkgs." + concatStringsSep "." example else example);
       });
 
-  /* Like mkPackageOption, but emit an mdDoc description instead of DocBook. */
-  mkPackageOptionMD = pkgs: name: extra:
-    let option = mkPackageOption pkgs name extra;
-    in option // { description = lib.mdDoc option.description; };
+  /* Alias of mkPackageOption. Previously used to create options with markdown
+     documentation, which is no longer required.
+  */
+  mkPackageOptionMD = mkPackageOption;
 
   /* This option accepts anything, but it does not produce any result.
 
@@ -347,11 +344,9 @@ rec {
   literalExample = lib.warn "literalExample is deprecated, use literalExpression instead, or use literalMD for a non-Nix description." literalExpression;
 
   /* Transition marker for documentation that's already migrated to markdown
-     syntax.
+     syntax. This is a no-op and no longer needed.
   */
-  mdDoc = text:
-    if ! isString text then throw "mdDoc expects a string."
-    else { _type = "mdDoc"; inherit text; };
+  mdDoc = lib.id;
 
   /* For use in the `defaultText` and `example` option attributes. Causes the
      given MD text to be inserted verbatim in the documentation, for when