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>

doc/api/assert.md

@@ -3,14 +3,11 @@
 > Stability: 3 - Locked
 
 The `assert` module provides a simple set of assertion tests that can be used to
-test invariants. The module is intended for internal use by Node.js, but can be
-used in application code via `require('assert')`. However, `assert` is not a
-testing framework, and is not intended to be used as a general purpose assertion
-library.
+test invariants.
 
 The API for the `assert` module is [Locked][]. This means that there will be no
-additions or changes to any of the methods implemented and exposed by
-the module.
+additions or changes to any of the methods implemented and exposed by the
+module.
 
 ## assert(value[, message])
 <!-- YAML