update the migration guide (#54185)

Author: ArcturusZhang

doc/LibrarySplit/MigrationGuideForSplit.md

@@ -94,4 +94,51 @@ using BarResource1 = BarPackage::Namespace.Resource1;
 
 var resourceFromFoo = new Resource1(); // From Foo v1.0.0
 var resourceFromBar = new BarResource1(); // From Bar v1.0.0
-```
+```
+
+## Changes Required in Mocking
+
+When migrating to the split libraries, only the mocking of **extension methods** that have moved to the new package is impacted. Mocking of instance methods and extension methods that remain in the original package will continue to work without changes.
+
+If you are mocking an extension method that has moved, you will notice that the corresponding method on the original mockable resource (e.g. `MockableFooSubscriptionResource`) is now marked as `Obsolete`. The compiler warning will provide a message pointing to the new method location in the new package.
+
+For example, if you were previously mocking an extension method on `SubscriptionResource` that was part of `Foo`, you might have had code like this:
+
+```csharp
+using Foo.Mocking;
+
+// ...
+
+var subscriptionMock = new Mock<SubscriptionResource>();
+var mockableSubscription = new Mock<MockableFooSubscriptionResource>();
+// GetResource1s is now Obsolete and will generate a warning
+mockableSubscription.Setup(x => x.GetResource1s(It.IsAny<CancellationToken>()))
+    .Returns(resource1CollectionMock.Object);
+
+subscriptionMock.Setup(x => x.GetCachedClient(It.IsAny<Func<ArmClient, MockableFooSubscriptionResource>>()))
+    .Returns(mockableSubscription.Object);
+```
+
+You might see a warning like this:
+> This method is obsolete. Call Bar.Mocking.MockableBarSubscriptionResource.GetResource1s instead.
+
+To fix this, you need to update your mocking code to use the mockable resource from the new package (e.g. `MockableBarSubscriptionResource`). Apart from updating the type name and the namespace, the method signatures remain the same, so no other logic changes are required.
+
+Here is an example of how to update your test code:
+
+```diff
+- using Foo.Mocking;
++ using Bar.Mocking;
+
+  // ...
+
+  var subscriptionMock = new Mock<SubscriptionResource>();
+- var mockableSubscription = new Mock<MockableFooSubscriptionResource>();
++ var mockableSubscription = new Mock<MockableBarSubscriptionResource>();
+  mockableSubscription.Setup(x => x.GetResource1s(It.IsAny<CancellationToken>()))
+      .Returns(resource1CollectionMock.Object);
+
+- subscriptionMock.Setup(x => x.GetCachedClient(It.IsAny<Func<ArmClient, MockableFooSubscriptionResource>>()))
++ subscriptionMock.Setup(x => x.GetCachedClient(It.IsAny<Func<ArmClient, MockableBarSubscriptionResource>>()))
+      .Returns(mockableSubscription.Object);
+```