doc: be less tentative about undefined behavior

In the documentation for the buffer module, change instances of "the
result is considered undefined behavior" to "the resulting behavior is
undefined". It's less ambiguous in meaning and less awkward
stylistically.

PR-URL: https://github.com/nodejs/node/pull/18091


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>