doc: Add Nixpkgs config options reference

doc/doc-support/default.nix

@@ -23,6 +23,14 @@ let
       <xsl:import href="${./parameters.xml}"/>
     </xsl:stylesheet>
   '';
+
+  # NB: This file describes the Nixpkgs manual, which happens to use module
+  #     docs infra originally developed for NixOS.
+  optionsDoc = pkgs.nixosOptionsDoc {
+    inherit (pkgs.lib.evalModules { modules = [ ../../pkgs/top-level/config.nix ]; }) options;
+    documentType = "none";
+  };
+
 in pkgs.runCommand "doc-support" {}
 ''
   mkdir result
@@ -30,6 +38,7 @@ in pkgs.runCommand "doc-support" {}
     cd result
     ln -s ${locationsXml} ./function-locations.xml
     ln -s ${functionDocs} ./function-docs
+    ln -s ${optionsDoc.optionsDocBook} ./config-options.docbook.xml
 
     ln -s ${pkgs.docbook5}/xml/rng/docbook/docbook.rng ./docbook.rng
     ln -s ${pkgs.docbook_xsl_ns}/xml/xsl ./xsl

doc/using/configuration.chapter.md

@@ -164,6 +164,14 @@ There are several ways to tweak how Nix handles a package which has been marked
 
     Note that `permittedInsecurePackages` is only checked if `allowInsecurePredicate` is not specified.
 
+### `config` Options Reference
+
+The following attributes can be passed in [`config`](#chap-packageconfig).
+
+```{=docbook}
+<include xmlns="http://www.w3.org/2001/XInclude" href="../doc-support/result/config-options.docbook.xml"/>
+```
+
 ## Modify packages via `packageOverrides` {#sec-modify-via-packageOverrides}
 
 You can define a function called `packageOverrides` in your local `~/.config/nixpkgs/config.nix` to override Nix packages. It must be a function that takes pkgs as an argument and returns a modified set of packages.

pkgs/top-level/config.nix

@@ -20,6 +20,11 @@ let
 
     /* Internal stuff */
 
+    # Hide built-in module system options from docs.
+    _module.args = mkOption {
+      internal = true;
+    };
+
     warnings = mkOption {
       type = types.listOf types.str;
       default = [];