modules: Update evalModules doc

lib/modules.nix

@@ -52,9 +52,32 @@ in
 
 rec {
 
-  /* Evaluate a set of modules.  The result is a set of two
-     attributes: ‘options’: the nested set of all option declarations,
-     and ‘config’: the nested set of all option values.
+  /*
+    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.
+
+      ‘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.
+
+      ‘_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.
+
      !!! Please think twice before adding to this argument list! The more
      that is specified here instead of in the modules themselves the harder
      it is to transparently move a set of modules to be a submodule of another