Clarify different geometries for extended CityObject

[Balázs Dukai]

The specs in 8.6 say that "If a new City Object contains other objects and requires different geometries [...]". What is meant by "different geometries" here? Is this related to the complex geometries #140 ?

[Hugo Ledoux]

I wrote this, I should know what is meant, but it's not clear :\

I meant this I assume, which should be clear:

Existing City Objects cannot be extended and have new types as children, eg it is not allowed to add a new City Object ”Balcony” to a ”Building”. Instead, a new type, eg ”FancyBuilding” should be created, which can have a "Balcony" as a potential children.