Clarify `Future` expectations

[kellytk]

From https://internals.rust-lang.org/t/future-and-its-assurance-of-completion/14542/11, originating with https://internals.rust-lang.org/t/future-and-its-assurance-of-completion/14542/10, the suggested change adds helpful information IMHO.

[taiki-e]

is typically expected to complete

I feel this explanation is unclear. Future is often defined to complete (=will eventually return Poll::Ready), but it is not uncommon for Future to not be polled until it is complete (=dropped before complete). Seems you are talking about the former, but I feel the explanation that this PR adds could easily be misunderstood as the latter.