doc: document fs.watchFile behaviour on ENOENT

When fs.watchFile encounters an ENOENT error, it invokes the given
callback with some error data. This caused an issue as it was different
behaviour than Node v0.10. Instead of changing this behaviour, document
it and add a test.

Ref: https://github.com/nodejs/io.js/issues/1745
Ref: https://github.com/nodejs/io.js/pull/2028


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/io.js/pull/2093