add callout for impact of changing standard attributes in Cognito

[cwomack]

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

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/