add clarity on how to handle confirmSignIn() and signIn() API responses for nextStep property

[cwomack]

Describe the content issue:
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We need to update the confirmSignIn() documentation to better clarify how to handle when this API returns CONFIRM_SIGN_IN_WITH_NEW_PASSWORD_REQUIRED as the nextStep. This is described in detail within the amplify-js issue #13598.

URL page where content issue is:
https://docs.amplify.aws/react/build-a-backend/auth/connect-your-frontend/sign-in/#using-the-signin-api