docs: added documentation to draw & update interfaces

Engine.Core/Systems/Abstract/IPostUpdate.cs

@@ -1,6 +1,12 @@
 namespace Syntriax.Engine.Core;
 
+/// <summary>
+/// Represents a <see cref="IBehaviour"/> to be notified after the update phase of the <see cref="IUniverse"/> occurs.
+/// </summary>
 public interface IPostUpdate : IBehaviour
 {
+    /// <summary>
+    /// Updates the state of the <see cref="IBehaviour"/> after the main update phase happens.
+    /// </summary>
     void PostUpdate();
 }