Use case for field `modified` needs to be specified

[jensstalder]

We need some documentation what exactly is this used for. Especially that relying on this field for changes is suspect to problems. Such as database changes might not map to the SwissRETS schema, making the updated field convoluted. But if we describe this better as a purely informational field, this will avoid confusion.

https://swissrets.ch/docs/noNamespace/complexType/exportType.properties.property.html

<property
  id = xs:string>
(address? ; author? ; availability ; bfs ; buildingZones? ; categories ; characteristics? ; created? ; development ; heating ; localizations ; minergieCertification ; ==>modified?<== ; parcelNumbers? ; prices? ; publishers ; referenceId ; seller ; type ; utilizations ; unitReferenceId? ; visualReferenceId?)
</property>