doc: document `buffer.from` returns internal pool buffer

Fixes: https://github.com/nodejs/node/issues/22139



Co-authored-by: Mritunjay Goutam <mritunjaygoutam2204@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/32703


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>