doc: update assert.rejects() docs with a validation function example

Spawned from my own struggle to use in https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1702#note_268452483

PR-URL: https://github.com/nodejs/node/pull/31271


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>