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.