Include a note about a scenario where forwarding will not occur

What does this MR do?

Include a scenario where the package registry request will not be forwarded to PyPi if the package is present but the version is missing. This will help to reduce time troubleshooting for those who may expect this scenario to forward the request to PyPi.

Related issues

Customer who experienced difficulty due to lack of specification: https://gitlab.zendesk.com/agent/tickets/332478

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Reviewer's checklist

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