lib/modules.nix: Deduplicate documentation

`file://./..` looks redundant, but makes the url clickable in vscode.

lib/modules.nix

@@ -63,35 +63,8 @@ let
           decls
       ));
 
-  /*
-    Evaluate a set of modules.  The result is a set with the attributes:
-
-      ‘options’: The nested set of all option declarations,
-
-      ‘config’: The nested set of all option values.
-
-      ‘type’: A module system type representing the module set as a submodule,
-            to be extended by configuration from the containing module set.
-
-            This is also available as the module argument ‘moduleType’.
-
-      ‘extendModules’: A function similar to ‘evalModules’ but building on top
-            of the module set. Its arguments, ‘modules’ and ‘specialArgs’ are
-            added to the existing values.
-
-            Using ‘extendModules’ a few times has no performance impact as long
-            as you only reference the final ‘options’ and ‘config’.
-            If you do reference multiple ‘config’ (or ‘options’) from before and
-            after ‘extendModules’, performance is the same as with multiple
-            ‘evalModules’ invocations, because the new modules' ability to
-            override existing configuration fundamentally requires a new
-            fixpoint to be constructed.
-
-            This is also available as a module argument.
-
-      ‘_module’: A portion of the configuration tree which is elided from
-            ‘config’. It contains some values that are mostly internal to the
-            module system implementation.
+  /* See https://nixos.org/manual/nixpkgs/unstable/#module-system-lib-evalModules
+     or file://./../doc/module-system/module-system.chapter.md
 
      !!! Please think twice before adding to this argument list! The more
      that is specified here instead of in the modules themselves the harder