docs: explain reading untyped values from AdditionalData

docs/handling-unknown-additional-data.md

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+# Reading unknown JSON properties from Microsoft Graph models
+
+The SDK stores response properties that are not represented by a generated model property in `AdditionalData`. In the Kiota based v5 SDK, JSON arrays and objects in this dictionary are represented by Kiota `UntypedNode` types, not `System.Text.Json.JsonElement`. Code that tests an additional-data value with `value is JsonElement` will therefore skip the value.
+
+For example, group delta responses can include the `members@delta` annotation. The following snippet processes the `members@delta` annotation for every group in one response page. It is not a complete delta-sync loop: follow every `@odata.nextLink`, process each page successfully, and persist the final `@odata.deltaLink` only after all changes have been applied:
+
+```csharp
+using Microsoft.Graph.Models;
+using Microsoft.Kiota.Abstractions.Serialization;
+
+foreach (var group in deltaResponse.Value ?? Enumerable.Empty<Group>())
+{
+    if (!group.AdditionalData.TryGetValue("members@delta", out var value) ||
+        value is not UntypedArray members)
+    {
+        continue;
+    }
+    foreach (var node in members.GetValue())
+    {
+        if (node is not UntypedObject member)
+        {
+            continue;
+        }
+
+        var properties = member.GetValue();
+        if (!properties.TryGetValue("id", out var idNode) ||
+            idNode is not UntypedString id)
+        {
+            continue;
+        }
+
+        var memberId = id.GetValue();
+        var isRemoved = properties.ContainsKey("@removed");
+
+        // Use memberId and isRemoved to apply the delta.
+    }
+}
+```
+
+Treat `@removed` as a marker: its presence means the member was removed. Delta consumers should apply additions and removals idempotently and persist the returned delta link only after successfully applying the page.
+
+The concrete value types are `UntypedArray`, `UntypedObject`, and `UntypedString` from Kiota abstractions. Avoid serializing the SDK model with `System.Text.Json.JsonSerializer` to inspect these values; that serializer does not define the Kiota `UntypedNode` representation.