Add AsyncDispatcher to the book

## 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