To improve the Sandcastle-Docs, and normal API navigation then default-value should be more visible:
/// <summary>
/// Gets or sets the layout that renders event ID.
/// </summary>
/// <value>Default: ${event-properties:item=EventId}</value>
public Layout<int> EventId { get; set; } = "${event-properties:item=EventId}";
Maybe also hint whether property-value is required (and not optional).