doc: update readFileSync in fs.md

* Updated fs.md stating fs.readFileAsync is platform specific
* Fix formatting of `note`s

PR-URL: https://github.com/nodejs/node/pull/12800
Refs: https://github.com/nodejs/node/issues/10962


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>