doc: Clarify the module.parent is set once

Per: 54a7f79a4a

Fixes: https://github.com/joyent/node/issues/6149

Originally submitted by @jorrit

Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2378

doc/api/modules.markdown

@@ -357,8 +357,7 @@ loading.
 
 * {Module Object}
 
-The module that required this one.
-
+The module that first required this one.
 
 ### module.children