Document at-rest encryption for swarm encryption keys

[mdlinville]

Task description

Document the ability to encrypt key-encryption keys used to encrypt Swarm intra-service communications, WALs, and secrets

See:

• At rest encryption: TLS keys and wrappers around the etcd wal/snap packages moby/swarmkit#1598
• Encryption at rest of manager keys and raft data moby/moby#27967
• https://docs.google.com/document/d/1YxMH2oIv-mtRcs1djRkm0ndLfiteo0UzBUFKYhLKYkQ/edit (proposal 1)

Project version(s) affected

Engine 1.13

cc/ @diogomonica @cyli

[mdlinville]

Fixed in #689

[docker-robott]

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