Describe how start works in the context of async

[lukewagner]

This PR fills in a TODO section in Async.md about what happens if an async-lifted function is used as a start function. It's pretty short, because it mostly just falls out from how async already works (but let me know if I'm forgetting to cover something).

To summarize, the idea is that start acts like a synchronous call, waiting until the async callee task.returns its value before continuing instantiation. But since async functions can keep running after task.return, so can these start-created tasks, "in the background". This appears to be quite useful as a solution for how language toolchains can support "background task"-type use cases without throwing out structured concurrency or the async call tree.