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>

doc/api/assert.markdown

@@ -2,10 +2,11 @@
 
     Stability: 3 - Locked
 
-The `assert` module provides a simple set of assertion tests that can be used
-to test invariants and implement unit tests. While the `assert` module is
-generally intended for internal use by Node.js itself, it can be used by user
-code calling `require('assert')`.
+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.
 
 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