docs(assert): clarify assert library use with monocdk. (#17202)

Using monocdk with the @aws-cdk/assert creates problems. I initially was going to create a PR to fix the problem it created (some circular dependency unravelling which JSON.stringify fails on, along with alarms not actually being found to be added to the stack I was testing), but instead I found this is what we should use instead, which this readme update would have saved me a lot of time troubleshooting.

This I believe should put this line right below the quote section in the top of both https://www.npmjs.com/package/@aws-cdk/assert and https://www.npmjs.com/package/@monocdk-experiment/assert

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/assert-internal/README.md

@@ -11,6 +11,8 @@
 > announced in the release notes. This means that while you may use them, you may need to update
 > your source code when upgrading to a newer version of this package.
 
+If using monocdk, use [@monocdk-experiment/assert](https://www.npmjs.com/package/@monocdk-experiment/assert) instead.
+
 ---
 
 <!--END STABILITY BANNER-->