[tsl:concurrency] Update AsyncValueRef documentation and add more implicit constructors
[tsl:concurrency] Update AsyncValueRef documentation and add more implicit constructors

Explicitly state in AsyncValueRef<T> documentation that it's an async version of absl::StatusOr<T> and that's how users should think about it.

Also add a test to verify that absl::StatusOr<absl::Status> and AsyncValueRef<absl::Status> have consistent implicit construction behavior.
