Describe how reply-to-all should work in the e-mail context

!122 (merged) introduces a new mechanism for key separation and key generation. that mechanism is useful for replying to a message without having keys for each participant.

Even with !122 (merged) merged, the draft only briefly mentions this capability, and doesn't describe how to use it.

It is probably out of scope to describe that in any detail, for any particular context like e-mail, but it would still be worth writing it down in the future somewhere, so that implementers of mail user agents have a concrete reference about how to use it.