Cleanup and document async.fs

[dsyme]

The process of fixing bug #11797 via #11799 made me go back and review async.fs end-to-end.

This does the following:

1. Very carefully documenting exactly when cancellation and trampoline/hijack checks are currently made

2. Fix Should async.TryFinally register the finally block before checking for cancellation? #8675, where a cancellation check was happening before a try/finally was being entered, causing resources to be potentially dropped on the floor if cancellation happens

3. Cleanup the code that does CancellationTokenRegistration (AwaitWaitHandle, AwaitBeginEnd etc.) to be more uniform

[Liminiens]

Maybe #8668 could also be addressed? 😇 If it still reproduces on main branch.

[dsyme]

Maybe #8668 could also be addressed? 😇 If it still reproduces on main branch.

I was surprised but as I mentioned in #8668 reducing the allocations by the technique I tried didn't actually improve performance. It might be I wasn't measuring correctly.

[dsyme]

This is ready. Best to review the changes with ignore-whitespace-changes on.

[KevinRansom]

Looks good.