Extend documentation about heartbeats and thresholds

[engelmi]

Please describe what information you were trying to find

In #921, a heartbeat signal from the controller to the agent as well as a configurable threshold in the agent has been introduced. We have now a two heartbeat signals and two thresholds in the components. Therefore, it would be good to extend the readthedoc documentation and explain the heartbeats, thresholds and how these correlate in more detail.