Add AsyncDispatcher to the book

[deontologician]

Description

Add some mention of AsyncDispatcher to the book, possibly with some considerations about when you might want to use it.

Motivation

Async is kind of an overloaded term. AsyncDispatcher uses a thread to dispatch, it doesn't integrate with futures or anything. It'd be nice to have some clear explanation that it's not related to futures/async/await and that some guidelines for when you might want to use it.

Drawbacks

Someone would have to spend time writing this up.

• Is it a breaking change? No
• Can it impact performance, learnability, etc? Should positively impact learnability

  Unresolved questions

  Not sure if this is undocumented because it's not used often (and so might not be worth documenting if it's going to be deprecated). I'm generally unaware of the context here

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Feel free to comment if this is still relevant.

[deontologician]

Any update on whether this might be documented?