doc: fix Buffer.from(object) documentation

Only strings are supported for objects supporting `Symbol.toPrimitive`.

PR-URL: https://github.com/nodejs/node/pull/33327


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>