These docstrings may need a rework

This is a collection of docstrings that may need some reformulating. I am happy to provide it, though would like to discuss the content beforhand.

• model.__init__: Update docstring
• aas module docstring and aas.Asset should have a note about how Assets are handled now
• aas.View: Resolve "Todo: What does this do exactly?" and with resolving, update docstring to be clear about what it does.
• base.Qualifiable: Unclear formulation
• base.HasExtension: Unclear explanation of attribute namespace_element_sets
• base.Referable.parent: Confusing by mentioning a Reference, even though it is actually a Namespace
• base.Reference.key: Unclear formulation
• base.UnexpectedTypeError: base.Reference does not have a method resolve()
• base.ValueReferencePair: Unclear formulation, does not have any reference to any other part of the meta-model
• provider.AbstractObjectProvider.get_identifiable:
  • Docstring says "find identifiable by id_short", parameter is of type base.Identifier though.
  • Missing docstring for ":param: identifier"
  • ":raises: KeyError: Mentioning Referable here, while correct, may be confusing to some. Maybe use Identifiable here?
• provider.AbstractObjectStore.update: Missing docstring
• submodel.OperationVariable: is in fact, not a SubmodelElement
• submodel.RelationshipElement: The mentioned subject-object relation is unclear to me. Disregard this, if this a concept I simply don't know
• Referable.display_name description is unclear
• base.Formula: If we don't support it anymore (in V30RC1, because formulas are not unambiguously identifiable), mention this somewhere where it makes sense
• aas.adapter.__init__: Update docstring
• adapter.json.json_deserialization: Second paragraph of module docstring seems outdated
• model.base.LangStringSet: Has no Entry in the documentation (Due to it being Dict[str, str]) and no class
• model.base.ValueList: Has not Entry in the documentation
• backend.__init__: Missing module docstring
• backend.couchdb: Missing module docstring