instruments/energy_measurement: add missing docs
instruments/energy_measurement: add missing docs

- Add missing descriptions to backends.
- Describe how to get instrument-specific parameter descriptions in the
  help for instrument_parameters param of EnergyMeasurementInstrument

ARM-software/workload-automation,setrofim/workload-automation,ARM-software/workload-automation,lisatn/workload-automation,lisatn/workload-automation,lisatn/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,setrofim/workload-automation,setrofim/workload-automation,lisatn/workload-automation