doc: improve assert strict-mode text

Simplify and clarify introductory material on `strict` mode.
Merge two single-sentence paragraphs into a single paragraph.
Replace "`strict mode`" with "`strict` mode".

PR-URL: https://github.com/nodejs/node/pull/28239


Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>