Add changelog entry + address PR review feedback

Author: noellie-velez

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -41,6 +41,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Changed
 
+- Improve performance of `NetworkObject`. (#3820)
 - Improve performance of `ParentSyncMessage`. (#3814)
 - Improve performance of `DestroyObjectMessage`. (#3801)
 - Improve performance of `CreateObjectMessage`. (#3800)

com.unity.netcode.gameobjects/Runtime/Core/NetworkObject.cs

@@ -2842,6 +2842,10 @@ public NetworkBehaviour GetNetworkBehaviourAtOrderIndex(ushort index)
             return ChildNetworkBehaviours[index];
         }
 
+        /// <summary>
+        /// The serialized representation of a NetworkObject.
+        /// Used for synchronizing clients on NetworkObject spawn.
+        /// </summary>
         internal struct SerializedObject
         {
             public uint Hash;