If there is interest in this, I'd like to attempt a PR for a sync/async variant of as_completed.

Note that the new docs are *much* clearer, so the first (documentation) problem from the description is now fixed. Although the documentation is still brief, it now contains the key pieces of information: 1) that the futures are actually run in parallel, and 2) that each yielded future produces the next result that becomes available. Neither was actually stated in the old docs (!), so this is a marked improvement.