doc: improve documentation of fs sync methods

Add links to async methods and make wording consistent.

PR-URL: https://github.com/nodejs/node/pull/21243
Refs: https://github.com/nodejs/node/issues/21197


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Jamie Davis <davisjam@vt.edu>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>