add callout for impact of changing standard attributes in Cognito

**Describe the content issue**:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?

What is needed for docs improvement is captured within `amplify-js` repo issue [#13724](https://github.com/aws-amplify/amplify-js/issues/13724)

Relevant Cognito docs - https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-settings-attributes.html#:~:text=%2B14325551212.-,preferred_username,-You%20can%20select

**URL page where content issue is**:
https://docs.amplify.aws/javascript/build-a-backend/auth/concepts/user-attributes/