doc,assert: rename "mode" to "assertion mode"

Rename "strict mode" in the assert module to "strict assertion mode".
This is to avoid confusion with the more typical meaning of "strict
mode" in ECMAScript.

This necessitates a corresponding change of "legacy mode" to "legacy
assertion mode".

PR-URL: https://github.com/nodejs/node/pull/31635


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>