fix: add section to clarify steps to update Cognito user email address with example

[kevinold]

Issue #, if available: https://github.com/aws-amplify/amplify-js/issues/987

Not ready for merge until https://github.com/aws-amplify/amplify-cli/pull/10773 has been merged.

Description of changes: Clarify how to update and verify a Cognito user email address change with full app example.

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