Add CAUTION to task batching doc

docs/task-batching.adoc

@@ -39,6 +39,15 @@ The above methods is not called automatically even when an error occurs in this
 This method runs in different thread from the `process(ProcessingContext<T>, T)` thread.
 |===
 
+[CAUTION]
+====
+Batch-flush is done in `BatchingProcessor`'s scheduled executor thread.
+This means the parallelism of flushing has to be controlled by `ProcessorScope`, not only by `decaton.partition.concurrency` config. i.e.:
+
+* Parallelize flushing per partition: `ProcessorScope.PARTITION`
+* Parallelize flushing per processor thread: `ProcessorScope.THREAD`
+====
+
 Before getting started with the example, let's create a `HelloTask` domain object which can be used to simulate the scenario we described:
 
 [source,java]