fix(auth): Fix incorrect statement about when userId is returned in AuthSignUpResult

[tylerjroach]

• PR title and description conform to Pull Request guidelines.
• [ ]

Issue #, if available:
#2775

Description of changes:
userId is not returned in AuthSignUpResult for confirmSignUp as it is not returned to us from the API. Cognito will only return userId on signUp calls.

We can't be more specific on this class, because AuthSignUpResult is in core, and a non-Cognito auth library may have different behavior as to when userId is and isn't returned.

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.