Add comment about DateTimeOffset binding from #2493 comment

https://github.com/CommunityToolkit/WindowsCommunityToolkit/issues/2493#issuecomment-424917060

Microsoft.Toolkit.Uwp.SampleApp/Data/DataGridDataItem.cs

@@ -149,6 +149,7 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.Data
 
         public uint Prominence { get; set; }
 
+        // You need to use DateTimeOffset to get proper binding to the CalendarDatePicker control, DateTime won't work.
         public DateTimeOffset First_ascent { get; set; }
 
         public string Ascents { get; set; }