describe DATASOURCE_TYPE & enable prompt flow even when AZURE_OPENAI_STREAM=true

[DOliana]

Motivation and Context

2 things:

• the documentation did not properly describe the variable DATASOURCE_TYPE
• prompt flow currently does not work, if AZURE_OPENAI_STREAM is set to true (probably the reason of Cannot use a deployed prompt flow in web app #934, since AZURE_OPENAI_STREAM defaults to true)

Contribution Checklist

• I have built and tested the code locally and in a deployed app
• I didn't break any existing functionality 😄

[abhahn]

Looks great to me, thanks for catching these issues and fixing them.

[abhahn]

Hi @DOliana , just wanted to let you know that I pulled your changes to my local and ran the integration tests locally and they all pass without any issues. I think the problem with the integration tests is related to this documented limitation of workflow secrets being unavailable when running workflows for PRs from forked repositories: https://docs.github.com/en/actions/security-guides/using-secrets-in-github-actions#using-secrets-in-a-workflow

I can bypass this check for now to get the PR merged.