Documenting about the two types of retry logics for 429 errors.

Please see this [#34 ](https://github.com/Azure/azure-openai-benchmark/issues/34)

The current documentation does not mention about the presence of the two retry logics in the code (& how they are coupled currently). However, it's important for the end user to be aware about it and hence, ideally it should be documented.

Start by reading linked issue #34 and reviewing the repository's current documentation for its treatment of 429 errors. Identify the two retry logics and how they are currently coupled, then document both for end users. Done means the documentation explains their presence and relationship clearly.

Written by the indexing model from the issue text.