Docs: Clarify whether durable purge-history can affect functions not being run locally

I need to do cleanup for my functions during local development, and it's unclear from the docs and my attempt to follow the source code whether running `func durable purge-history` can ever affect functions running in Azure.

I'd appreciate if these could be documented:

- Any risks of `purge-history` affecting non-local Durable Function instances.
- How to identify all Functions that would be affected by `purge-history`, so I can sanity-check before doing it.

Start with the `func durable purge-history` command documentation and follow the source-code path referenced in the issue. Document whether local use can affect Azure-hosted instances, explain the risks, and describe how users can identify every affected Function before running it.

Written by the indexing model from the issue text.