Diagram: update dataItem description (DevExpress#6901)

Author: vladaskorohodova

api-reference/10 UI Components/dxDiagram/7 Interfaces/dxDiagramShape/dataItem.md

@@ -0,0 +1,5 @@
+While dragging a shape on the canvas, **dataItem** might be `undefined`. Implement a null check.
+
+#include btn-open-demo with {
+    href: "https://js.devexpress.com/Demos/WidgetsGallery/Demo/Diagram/CustomShapesWithTemplatesWithEditing/"
+}