doc: linkify remaining references to fs.Stats object

One reference to the `fs.Stats object` is linkified, while two other
similarly made references to the object are not linkified.
Linking the remaining references makes sense due to similar context.

PR-URL: https://github.com/nodejs/node/pull/6485


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>