doc: tidy AssertionError text

Several minor improvements to text. Remove some repetition.

PR-URL: https://github.com/nodejs/node/pull/28255


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>