Document Azure Managed async client fix

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 7bc5f597-1596-460d-bfe6-738676344ef0

Author: andystaples

durabletask-azuremanaged/CHANGELOG.md

@@ -21,6 +21,9 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 - Improved async access token refresh concurrency handling to avoid duplicate
   refresh operations under concurrent access, matching the existing sync
   behavior.
+- Fixed `AsyncDurableTaskSchedulerClient` failing during construction when no
+  current event loop was set. Its async gRPC channel is now created on first
+  use and bound to the event loop performing the request.
 
 ## v1.8.0