Add description config field on Tango devices.

This adds a "description" field to stuff that have corresponding Tango devices. It just maps directly to the "description" property. It's a standard Tango feature that the device description can be overridden with this property, see https://tango-controls.readthedocs.io/en/latest/development/advanced/reference.html#the-device-description-field

I think this would be a very nice way to document the purpose of elements etc, since it would be available via the controlsystem too.

Note that some things like attributes and measurement groups already had a "description" field, so this is consistent.