doc: improve Buffer.allocUnsafeSlow() and related

* Fixed "cleanup" being misused as a verb
* "Use of Foo should only be used" construction changed to "Foo should
  only be used..."
* Otherwise-unmentioned "`Persistent`" changed to more understandable
  "persistent"
* remove an instance of unnecessary italics
* wrap at 80 characters

Change all "initialize a...instance to zeroes" to say "with zeroes"
instead. Previously, both formulations appeared.

PR-URL: https://github.com/nodejs/node/pull/19742


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>