doc: add clarification on birthtime in fs stat

Clarifies the possibility of birthtime in the fs stat
object being greater than atime or mtime when not available
in the filesystem (see issue for further info).

Fixes: https://github.com/nodejs/node/issues/2222
PR-URL: https://github.com/nodejs/node/pull/5479


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