doc: add more internal links to fs.Stats object

PR-URL: https://github.com/nodejs/node/pull/9345
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/fs.md

@@ -877,7 +877,7 @@ added: v0.1.95
 
 * `fd` {Integer}
 
-Synchronous fstat(2). Returns an instance of `fs.Stats`.
+Synchronous fstat(2). Returns an instance of [`fs.Stats`][].
 
 ## fs.fsync(fd, callback)
 <!-- YAML
@@ -1076,7 +1076,7 @@ added: v0.1.30
 
 * `path` {String | Buffer}
 
-Synchronous lstat(2). Returns an instance of `fs.Stats`.
+Synchronous lstat(2). Returns an instance of [`fs.Stats`][].
 
 ## fs.mkdir(path[, mode], callback)
 <!-- YAML