DataGrid: add a note about _DX_KEY_ (DevExpress#7131) (DevExpress#7135)

Author: vladaskorohodova

api-reference/10 UI Components/dxDataGrid/1 Configuration/editing/changes/key.md

@@ -0,0 +1,12 @@
+The **key** property supports both simple and composite keys.
+
+    <!-- tab: JavaScript -->changes.push({
+        type: 'update',
+        data: { FirstName : 'name1', LastName: 'name2' }
+
+        key: 1 // simple key
+        // or ...
+        key: {ID: 1, SecondID: 2} // composite key
+    });
+
+[important] In batch [editing mode](/Documentation/ApiReference/UI_Components/dxDataGrid/Configuration/editing/#mode), the "insert" operation generates a temporary GUID-like key that starts with the `_DX_KEY_` prefix. Avoid basing application logic on this key.