This is a tracking bug for a dependency of the maas MIR (bug #961344).
MAAS is something that enterprises will use to deploy services and machines that are reachable over the network so highlevel documentation would aid in more peer review and help adoption in the enterprise. For the design document, just some high level stuff with a few diagrams so that people understand the architecture, network topologies, how nodes are added/deleted/provisioned, the role of users, etc. The security design document would complement this and mention how ssh access works, the various daemon that are running on the server, the technologies used by the server, admin vs non-admin users, a node's default configuration, etc. The two could be in one document and could be in a man page or some part of the official documentation.