Detailed documentation on Durable's use of generators in JS

The `durable-functions` module makes use of JavaScript's generators to simulate the partial replay behavior required by orchestrator functions. The result is specialized use of and certain limitations around the `yield` keyword within orchestrators.

Current documentation makes sparse mention of these requirements ([here](https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-checkpointing-and-replay#orchestrator-code-constraints) and [here](https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-concepts#event-sourcing-checkpointing-and-replay)), leading to confusion. We should expand our documentation to include a language-specific section covering, but not limited to:

- [ ] how Durable Functions for JavaScript uses generators
- [ ] limitations as a result of this special usage

Start with the existing orchestrator code constraints and event-sourcing/checkpointing-and-replay documentation linked in the issue, then inspect the durable-functions module's generator usage. Done means the documentation includes a language-specific explanation of how JavaScript generators support replay and the resulting yield limitations.

Written by the indexing model from the issue text.