Update/add docstrings (PEP257)

homeassistant/components/sensor/__init__.py

homeassistant/components/sensor/apcupsd.py

homeassistant/components/sensor/arduino.py

homeassistant/components/sensor/arest.py

homeassistant/components/sensor/bitcoin.py

homeassistant/components/sensor/command_sensor.py

homeassistant/components/sensor/cpuspeed.py

homeassistant/components/sensor/demo.py

homeassistant/components/sensor/dht.py

homeassistant/components/sensor/dweet.py

homeassistant/components/sensor/ecobee.py

homeassistant/components/sensor/efergy.py

homeassistant/components/sensor/eliqonline.py

homeassistant/components/sensor/forecast.py

homeassistant/components/sensor/glances.py

homeassistant/components/sensor/isy994.py

homeassistant/components/sensor/mfi.py

homeassistant/components/sensor/modbus.py

homeassistant/components/sensor/mqtt.py

homeassistant/components/sensor/mysensors.py