doc: stronger suggestion for userland assert

Fixes: https://github.com/nodejs/node/issues/4532
PR-URL: https://github.com/nodejs/node/pull/4535


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>