doc: improve assert legacy text

This changes a sentence fragment into a full sentence and provides a few
other minor improvements.

PR-URL: https://github.com/nodejs/node/pull/19622


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>