nixos/documentation: Make extraModules configurable

nixos/modules/misc/documentation.nix

@@ -7,9 +7,6 @@ let
   cfg = config.documentation;
   allOpts = options;
 
-  /* Modules for which to show options even when not imported. */
-  extraDocModules = [ ../virtualisation/qemu-vm.nix ];
-
   canCacheDocs = m:
     let
       f = import m;
@@ -23,7 +20,7 @@ let
 
   docModules =
     let
-      p = partition canCacheDocs (baseModules ++ extraDocModules);
+      p = partition canCacheDocs (baseModules ++ cfg.nixos.extraModules);
     in
       {
         lazy = p.right;
@@ -236,6 +233,14 @@ in
         '';
       };
 
+      nixos.extraModules = mkOption {
+        type = types.listOf types.raw;
+        default = [];
+        description = ''
+          Modules for which to show options even when not imported.
+        '';
+      };
+
       nixos.options.splitBuild = mkOption {
         type = types.bool;
         default = true;