doc,test: documents behaviour of non-existent file

As per the discussion in
https://github.com/nodejs/io.js/pull/2093#discussion_r34343965, this
patch documents the behavior of calling fs.watchFile() with a path that
does not yet exist.

This patch also includes a test which checks if a file not present, the
callback is invoked at least once and if the file is created after
the callback is invoked, it will be invoked again with new stat
objects.

PR-URL: https://github.com/nodejs/io.js/pull/2169


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>