add a note to the Operation Store AR guide about test environments

By default, `last_used_at` timestamps are updated every 5 seconds in a background thread. However, in a test environment, this can cause some surprising and unintended side effects when the background thread update is delayed until after a test finishes, and the test transaction has been rolled back.

Author: agrobbin

guides/operation_store/active_record_backend.md

@@ -93,3 +93,10 @@ use GraphQL::Pro::OperationStore, update_last_used_at_every: 1 # seconds
 ```
 
 To update that column inline each time an operation is accessed, pass `0`.
+
+**Note:** It is recommended to set this to `0` in test environments, to avoid delayed updates in another thread that can cause intermittent test hangs and failures. For example:
+
+```ruby
+# Update immediately in Test, wait 5 seconds in other environments:
+use GraphQL::Pro::OperationStore, update_last_used_at_every: Rails.env.test? ? 0 : 5
+```