Document experimental async support

[tim-schilling]

As per our November 2024 meeting, to release 5.0.0 we'd like to formalize the async support. However, we're unsure of how stable the integration is. Considering we can't support concurrent requests properly (requires the serializable branch to be merged), it makes sense to indicate that async is experimental.

Let's document that the library's support of async is experimental. I think any place that mentions async or asynchronous should be reviewed for language.