doc: add callback function signatures in fs.md

PR-URL: https://github.com/nodejs/node/pull/13424
Refs: https://github.com/nodejs/node/issues/11135


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>