funnel: clarify Windows support

[buchanae]

There's no good reason (that I'm aware of) to limit access to Funnel on Windows, and there are benefits. We don't currently have a good picture of what the existing issues are. I suspect it's mainly just issues with local file storage, and maybe some docker exec issues, but I would imagine most things are portable.

In particular, this code should be reconsidered: #291 (review)