[Amend SE-0526] Clarify Deadline interactions with Cancellation Shields

Hello everyone, While working on an upcoming proposal (task cancellation scopes) myself and @FranzBusch have realized another interaction of deadlines and cancellation shields we'd like to clarify and codify. Since deadlines have not yet shipped, and will be part of 6.5, we propose to amend the existing proposal rather than add another one -- this way the source of truth remains in one proposal. In one sentence, we're proposing that to achieve consistent cancellation behavior with cancellation...