improve signUp API docs to clearly state options that can be passed in

[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?

See JS Issue #13620. Docs for signUp API could be more detailed on what can be passed in (i.e. username, email, etc.).

URL page where content issue is: https://docs.amplify.aws/react/build-a-backend/auth/connect-your-frontend/sign-up/