doc: improve zero-fill-buffers text

* improve text for easier comprehension
* clarify that performance impact is *negative*
* remove superfluous "either" (should only be used when there are 2
  options anyway)
* remove superfluous italics
* line wrap at 80 chars

PR-URL: https://github.com/nodejs/node/pull/19623


Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>