Add serialisation/deserialisation explanation

osu.Desktop/LegacyIpc/LegacyIpcMessage.cs

@@ -2,11 +2,24 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using osu.Framework.Platform;
+using Newtonsoft.Json.Linq;
 
 namespace osu.Desktop.LegacyIpc
 {
     /// <summary>
     /// An <see cref="IpcMessage"/> that can be used to communicate to and from legacy clients.
+    /// <para>
+    /// In order to deserialise types at either end, types must be serialised as their <see cref="System.Type.AssemblyQualifiedName"/>,
+    /// however this cannot be done since osu!stable and osu!lazer live in two different assemblies.
+    /// <br />
+    /// To get around this, this class exists which serialises a payload (<see cref="LegacyIpcMessage.Data"/>) as an <see cref="System.Object"/> type,
+    /// which can be deserialised at either end because it is part of the core library (mscorlib / System.Private.CorLib).
+    /// The payload contains the data to be sent over the IPC channel.
+    /// <br />
+    /// At either end, Json.NET deserialises the payload into a <see cref="JObject"/> which is manually converted back into the expected <see cref="LegacyIpcMessage.Data"/> type,
+    /// which then further contains another <see cref="JObject"/> representing the data sent over the IPC channel whose type can likewise be lazily matched through
+    /// <see cref="LegacyIpcMessage.Data.MessageType"/>.
+    /// </para>
     /// </summary>
     /// <remarks>
     /// Synchronise any changes with osu-stable.

osu.Desktop/LegacyIpc/LegacyTcpIpcProvider.cs

@@ -36,6 +36,7 @@ namespace osu.Desktop.LegacyIpc
                     logger.Add("Processing legacy IPC message...");
                     logger.Add($"    {msg.Value}", LogLevel.Debug);
 
+                    // See explanation in LegacyIpcMessage for why this is done this way.
                     var legacyData = ((JObject)msg.Value).ToObject<LegacyIpcMessage.Data>();
                     object value = parseObject((JObject)legacyData!.MessageData, legacyData.MessageType);