Reword derivation of overallStatus

[Pete-Cordell]

I thought the approach Stefan took in his PowerPoint slides about how overallStatus is derived is clearer. Here's some Markdown that attempts to capture that:

Devices MUST follow the rules below when mapping the domain statuses into the combined overallStatus:

• When the Receiver is Inactive the overallStatus is set to Inactive
• On Receiver Activation the Receiver monitor enters a stabilisation period in which the overallStatus is set to Healthy and remains Healthy for the duration specified by the statusReportingDelay property. (This allows for devices to go through a period of instability when connecting to new streams without generating spurious alarms for an operator.)
• Once the stabilisation period has ended and until the Receiver becomes Inactive the following algorithm applies:
  • If all statuses are either Healthy (or equivalent) or Unused the overallStatus is set to Healthy
  • Otherwise if any status is Unhealthy (or equivalent) the overallStatus is set to Unhealthy
  • Otherwise the overallStatus is set to PartiallyHealthy