docs: document change for clustercache affecting tests

Author: chrischdi

docs/book/src/developer/providers/migrations/v1.9-to-v1.10.md

@@ -21,5 +21,6 @@ maintainers of providers and consumers of our Go API.
   - `E2EConfig.GetVariable` is now `E2EConfig.MustGetVariable`
   - `E2EConfig.GetInt64PtrVariable` is now `E2EConfig.MustGetInt64PtrVariable`
   - `E2EConfig.GetInt32PtrVariable` is now `E2EConfig.MustGetInt32PtrVariable`
+- Using the package `sigs.k8s.io/cluster-api/controllers/clustercache` in tests using envtest may require a change to properly shutdown a running clustercache. Otherwise teardown of envtest might time out and lead to failed tests. (xref [#11757](https://github.com/kubernetes-sigs/cluster-api/pull/11757))
 
 ### Suggested changes for providers