doc: remove assertions about assert

The assert docs have some language that suggests that we don't want bug
fixes. We do. Send in bug fixes, please. (Just no new API features.)
We'd love to not have assert in core at all, but that ship has sailed.
It's here to stay. Let's at least make it not have surprising behaviors.
Because we want good things for our users.

PR-URL: https://github.com/nodejs/node/pull/11113


Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>