doc: improve Buffer() text

Rewording, punctuation, consistent sentence structure and italics, wrap
section at 80 characters.

PR-URL: https://github.com/nodejs/node/pull/19567


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>