doc: improve fs api descriptions

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

I have heard from many people how bad our docs are for people not familiar with some standard POSIX functions.

https://twitter.com/sindresorhus/status/939823222517850112 is just one example. I've also been told this in person on multiple occasions.

I think it is time for us to expand and improve our documentation to make it as useful as possible to all developers.

I know this isn't a large change but I do think it is an improvement over what we currently have. I'm also open to wording/description suggestions.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc