[Docs] Docs on how Propeller ensures a task is executed exactly once

[sonjaer]

Description

I was curious how Propeller ensures a task is executed exactly once? I was curious since we have non idempotent tasks that we wouldn't want to run twice. Is there any instance when this could not be true? When deploying a new Propeller or error writing to etcd? Is this something that could be added to the Propeller Architecture docs here

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