Documenting about MAX_RETRY_SECONDS

MAX_RETRY_SECONDS is also an important constant in the code for specifying the duration to stop the throttling retries.
However, there's no mention about same in the documentation.

Ideally, it should also be mentioned about in the documentation so that the user is aware about it.
The documentation should talk about the reason for keeping this current hardcoded value of 5s & it's usage (for e.g. in retry logic and backoff logic) etc.

Relates to - [#36 ](https://github.com/Azure/azure-openai-benchmark/issues/36)

Locate MAX_RETRY_SECONDS in the repository and read the surrounding retry and throttling logic first. Update the relevant documentation to explain its hardcoded 5-second value, its purpose, and its use in retry or backoff behavior; done means users can understand why it exists and how it affects retries.

Written by the indexing model from the issue text.