Async guidelines aren't clear that they are showing how to implement memoization manually, rather than behavior of the memoize attribute

[yschimke]

https://docs.hhvm.com/hack/async/guidelines#memoization-may-be-good.-but-only-awaitables

"The <<__Memoize>> attribute does the right thing" but code samples don't show any examples of <<__Memoize>>