Improve documentation around Cognito triggers tied to Auth flows

[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?

More detailed description of content gap and context are captured within amplify-js repo issue #11565, specifically this comment .

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