doc: revise --zero-fill-buffers text in buffer.md

There was an unclear sentence fragment that needed fixing, so I edited
the entire paragraph for clarity. I also removed irrelevant information
about behavior before Node.js 8.0.0. That version of Node.js is no
longer supported and these docs will never apply to 8.0.0. (At the time
of this writing, 10.x is the oldest supported line, and so changes to
the docs will never be backported farther than the 10.x docs.)

PR-URL: https://github.com/nodejs/node/pull/32019


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>