Document single_threaded_async feature

[mxgrey]

Before proceeding, is there an existing issue or discussion for this?

• I have done a search for similar issues and discussions.

Description

Until web assembly can support threading, users that target web assembly will need to activate the single_threaded_async feature.

I believe we shouldn't automatically turn on the single_threaded_async behavior for wasm targets because

1. web assembly will probably support multi-threading before very long
2. users may have reasons for wanting the single-threaded execution even without wasm

so we'll just keep it as its own feature, but we should provide clear documentation on when, how, and why to use it.