Document the WorkerActivity state machine

The allowed transitions between WorkerActivity states should be documented somewhere.

The started → started update is only allowed after the worker_activity_timeout expires to prevent the same task from running twice on the same element.

Original PlantUML diagram:

@startuml
state queued
hide empty description
queued --> started
started --> started
error --> started
error --> queued
started --> error
started --> processed
@enduml