doc: fix some recent nits in fs.md

* Fix sorting of sections.
* Fix sorting of bottom references.
* Wrap lines at 80 chars.
* Fix a heading level.

PR-URL: https://github.com/nodejs/node/pull/29906


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>