Pins the three properties the revocation rests on and cannot check
against a store: that only `Destroyed` revokes (a revocation on
`Offline` or `Paused` would give an agent that stops and never
restarts), that a 404 is absence while a 403 stays a failure, and that
the delete addresses `secret/metadata/` -- the path that takes every
version, which is the string the grant has to match.
docs/swarm/credentials.md gains the revocation section and its table
cell stops describing the deletion as something an operator does by
hand.
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