doc: properly document AssertionError

The AssertionError was always exposed but never properly documented.
This explains how it is used and what options it accepts.

PR-URL: https://github.com/nodejs/node/pull/19724


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>