doc: add missing changes entry

The `util.format()` behavior changed recently. Add the changes entry
to document the new BigInt behavior.

PR-URL: https://github.com/nodejs/node/pull/24758


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>