Documenting reference for header values used.

It would be good to document the source/reference of all external header values which we are relying upon.

FileName: oairequester.py

For example,
It's difficult to find the reference of "azure-openai-deployment-utilization" on a quick web/Azure documentation search.

As an example, following documentation link can be ref. of headers like "retry-after-ms"
https://learn.microsoft.com/en-us/azure/ai-services/openai/concepts/provisioned-throughput

Start with oairequester.py and inventory the external header values used there, including "azure-openai-deployment-utilization" and "retry-after-ms". Find the authoritative Azure OpenAI references for each value and document those sources; the work is done when every external header value in the file has a reference.

Written by the indexing model from the issue text.