doc: improve strict assertion mode color text

Minor improvements to the text about colors in strict assertion mode:

* Consolidate the two paragraphs into one
* Pluralize "variables"
* Remove unnecessary article "the"
* Change "doc" to "documentation"

PR-URL: https://github.com/nodejs/node/pull/31703


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