This method is called after this document of a part of this document has been modified. To avoid confusion, this method and any method that it calls should not modify the document. Besides invoking all Changed event handlers, this also calls GoUndoManager.DocumentChanged if there is an UndoManager and sets IsModified to true, unless SkipsUndoManager is true. This method calls UpdateDocumentBounds if an object is inserted into a layer or it its bounds change. An insertion or removal of an object from a layer, if MaintainsPartID is true and the object is an IGoIdentifiablePart, will also update the object's IGoIdentifiablePart.PartID and the document's internal table of part ID mappings.