model: Add docstring for intializers, including argument description
- The argument description should be copied from the (super-)class' attribute description, extended by " (from <Classname>)"
- It should be mentioned, what should be done typically with the object after constructing it (e.g. "Should be added to a Submodel or SubmodelElementCollection via
add_element()
"). Probably we should add a TODO here, until the interface for adding elements etc. is fixed.