Clarity on v5 to v6 Auth migration docs for `CUSTOM_WITHOUT_SRP`

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

Details within JS Issue [#13049](https://github.com/aws-amplify/amplify-js/issues/13049), but essentially need to add clarity within the v5 to v6 migration guides for Auth that when someone is trying to use custom challenges for passwordless sign in, they need to update their API calls as shown [here](https://github.com/aws-amplify/amplify-js/issues/13049#issuecomment-1964170864).

**URL page where content issue is**:

https://docs.amplify.aws/react/build-a-backend/auth/auth-migration-guide/#authsignin