Add note about registering vm list types. (#15279)

Fixes https://github.com/openxla/iree/issues/15251

skip-ci: comment-only change
diff --git a/runtime/src/iree/vm/list_test.cc b/runtime/src/iree/vm/list_test.cc
index 6a9a3a0..1aa994a 100644
--- a/runtime/src/iree/vm/list_test.cc
+++ b/runtime/src/iree/vm/list_test.cc
@@ -156,6 +156,8 @@
 static iree_vm_instance_t* instance = nullptr;
 struct VMListTest : public ::testing::Test {
   static void SetUpTestSuite() {
+    // Note: VM instance creation registers list types, which is required before
+    // using the list APIs.
     IREE_CHECK_OK(iree_vm_instance_create(IREE_VM_TYPE_CAPACITY_DEFAULT,
                                           iree_allocator_system(), &instance));
     RegisterRefTypes(instance);