doc: improve assert documentation

This adds a example to `assert.throws` to document checking against
error instances. It also makes it clearer what some arguments can
receive as input.

PR-URL: https://github.com/nodejs/node/pull/19885


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>