Added documentation to communicate limitations of when using the

AsyncItemProcessor.

BATCH-2371

Author: mminella

spring-batch-integration/src/main/java/org/springframework/batch/integration/async/AsyncItemProcessor.java

@@ -33,11 +33,19 @@
  * background. To allow for background processing the return value from the
  * processor is a {@link Future} which needs to be unpacked before the item can
  * be used by a client.
+ *
+ * Because the {@link Future} is typically unwrapped in the {@link ItemWriter},
+ * there are lifecycle and stats limitations (since the framework doesn't know
+ * what the result of the processor is).  While not an exhaustive list, things like
+ * {@link StepExecution#filterCount} will not reflect the number of filtered items
+ * and {@link org.springframework.batch.core.ItemProcessListener#onProcessError(Object, Exception)}
+ * will not be called.
  * 
  * @author Dave Syer
  * 
  * @param <I> the input object type
  * @param <O> the output object type (will be wrapped in a Future)
+ * @see AsyncItemWriter
  */
 public class AsyncItemProcessor<I, O> implements ItemProcessor<I, Future<O>>, InitializingBean {