Improve documentation around Cognito triggers tied to Auth flows

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

More detailed description of content gap and context are captured within `amplify-js` repo issue [#11565](https://github.com/aws-amplify/amplify-js/issues/11565), specifically [this comment ](https://github.com/aws-amplify/amplify-js/issues/11565#issuecomment-1624603294).

**URL page where content issue is**:
https://docs.amplify.aws/react/build-a-backend/auth/advanced-workflows/#lambda-triggers