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.

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