docs(assertions): add more detail about migrating from the old assert…

… library (#25618)

See [rich diff](https://github.com/aws/aws-cdk/pull/25618/files?short_path=3ee803f#diff-3ee803f6f019fbebe51ba0803fc63385b39c813e4f7aa1ef05b2c4b384e164e8).

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packages/aws-cdk-lib/assertions/README.md

@@ -1,8 +1,8 @@
 # Assertions
 
 
-If you're migrating from the old `assert` library, the migration guide can be found in
-[our GitHub repository](https://github.com/aws/aws-cdk/blob/v1-main/packages/@aws-cdk/assertions/MIGRATING.md).
+If you're migrating from the old `@aws-cdk/assert` library, first use this migration guide to migrate from `@aws-cdk/assert` to `@aws-cdk/assertions` found in
+[our GitHub repository](https://github.com/aws/aws-cdk/blob/v1-main/packages/@aws-cdk/assertions/MIGRATING.md). Then, you can migrate your application to AWS CDK v2 in order to use this library using [this guide](https://docs.aws.amazon.com/cdk/v2/guide/migrating-v2.html).
 
 Functions for writing test asserting against CDK applications, with focus on CloudFormation templates.