doc: update language regarding key stretching

Update the docs to provide clearer instructions regarding the exact
scope of the use (and re-use) of an IV, stating the instructions
explicitly with greater clarity.

PR-URL: https://github.com/nodejs/node/pull/19810
Fixes: https://github.com/nodejs/node/issues/19748


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>