Missing documentation for CDK based custom resource limitations

**Is your feature request related to a problem? Please describe.**
I didn't realize that you can't use CDK to create custom lambda resources. This is a big limitation and deserves to be mentioned in the docs.

Some related issues:
https://github.com/aws-amplify/amplify-cli/issues/9055
https://github.com/aws-amplify/docs/issues/4846

**Describe the solution you'd like**
Please update the documentation here https://docs.amplify.aws/cli/custom/cdk/ to reflect this limitation.

**Describe alternatives you've considered**
NA

**Additional context**

Open the custom CDK documentation at https://docs.amplify.aws/cli/custom/cdk/ and review how custom resources are described. Confirm the limitation against the related issues, then update that page so the restriction on creating custom Lambda resources with CDK is explicit; the documentation should clearly communicate the limitation to users.

Written by the indexing model from the issue text.