doc: improve fs api descriptions

This improves the api descriptions for fs.chown, fs.chmod, and fs.mkdir
along with their *Sync counterparts.

PR-URL: https://github.com/nodejs/node/pull/17679


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>