We could use an accompanying document to the existing docs/futures_and_promises.md user-facing guide that delves into the implementation of the futures library. Ideally, no one outside of SP should ever need to reference such a document; mostly, this is for us to reference and pass down institutional knowledge. However, realistically, other teams run into issues related to futures that require a deeper understanding of how they work to diagnose effectively, and so they could also benefit from such documentation.