doc: suggest alternatives to deprecated APs

At the moment users who want to use `fs.exists` get a warning that the
method is deprecated but do not get offered an alternative in the page.
This PR suggests `fs.stat` and `fs.access` as alternatives while
keeping the warning about the use case in place.

Fixes: https://github.com/iojs/io.js/issues/1002
PR-URL: https://github.com/iojs/io.js/pull/1007


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Christian Tellnes <christian@tellnes.no>