doc: note v15.0.0 changed default --unhandled-rejections=throw

PR-URL: https://github.com/nodejs/node/pull/36361


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Rich Trott <rtrott@gmail.com>