Reapply to futures.qbk the changes I'd previously made to future.qbk.
When explanatory text was moved from future.qbk to futures.qbk, my changes to
that text failed to apply to Oliver's trimmed-down version of future.qbk, and
I ended up discarding them. But since that text migrated to futures.qbk, the
edits are still relevant. Re-edit them into futures.qbk by hand.
Review pass through new packaged_task.qbk and promise.qbk.