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

**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](https://github.com/aws-amplify/amplify-js/issues/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/