Document most core interfaces

Oqtane.Shared/Interfaces/IDeletable.cs

@@ -1,11 +1,25 @@
-using System;
+using System;
 
 namespace Oqtane.Models
 {
+    /// <summary>
+    /// Audit information for things than can be deleted.
+    /// </summary>
     public interface IDeletable
     {
+        /// <summary>
+        /// <see cref="User"/> who deleted this object.
+        /// </summary>
         string DeletedBy { get; set; }
+
+        /// <summary>
+        /// Timestamp when it was deleted.
+        /// </summary>
         DateTime? DeletedOn { get; set; }
+
+        /// <summary>
+        /// If something is deleted, this will be true.
+        /// </summary>
         bool IsDeleted { get; set; }
     }
 }