Adds XML header docs indicating usage of options on NuCacheSerializerType (#19555)

Adds XML header docs indicating usage of options on NuCacheSerializerType.

src/Umbraco.Core/Configuration/Models/NuCacheSerializerType.cs

@@ -4,10 +4,23 @@
 namespace Umbraco.Cms.Core.Configuration.Models;
 
 /// <summary>
-///     The serializer type that nucache uses to persist documents in the database.
+///     The serializer type that the published content cache uses to persist documents in the database.
 /// </summary>
 public enum NuCacheSerializerType
 {
-    MessagePack = 1, // Default
+    /// <summary>
+    /// The default serializer type, which uses MessagePack for serialization.
+    /// </summary>
+    MessagePack = 1,
+
+    /// <summary>
+    /// The legacy JSON serializer type, which uses JSON for serialization.
+    /// </summary>
+    /// <remarks>
+    /// This option was provided for backward compatibility for the Umbraco cache implementation used from Umbraco 8 to 14 (NuCache).
+    /// It is no longer supported with the cache implementation from Umbraco 15 based on .NET's Hybrid cache.
+    /// Use the faster and more compact <see cref="MessagePack"/> instead.
+    /// The option is kept available only for a more readable format suitable for testing purposes.
+    /// </remarks>
     JSON = 2,
 }