Update documentation about serializable keys in the execution context

Resolves #4457

Author: hwan33

spring-batch-docs/modules/ROOT/pages/domain.adoc

@@ -534,6 +534,12 @@ As noted in the comment, `ecStep` does not equal `ecJob`. They are two different
 `ExecutionContexts`. The one scoped to the `Step` is saved at every commit point in the
 `Step`, whereas the one scoped to the Job is saved in between every `Step` execution.
 
+NOTE: In the `ExecutionContext`, all non-transient entries must be `Serializable`.
+Proper serialization of the execution context underpins the restart capability of steps and jobs.
+Should you use keys or values that are not natively serializable, you are required to
+employ a tailored serialization approach. Failing to serialize the execution context
+may jeopardize the state persistence process, making failed jobs impossible to recover properly.
+
 [[jobrepository]]
 == JobRepository
 

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/ExecutionContext.java

@@ -30,8 +30,9 @@
  * that allows optionally for type safety on reads. It also allows for dirty checking by
  * setting a 'dirty' flag whenever any put is called.
  * <p>
- * Note that putting <code>null</code> value is equivalent to removing the entry for the
- * given key.
+ * Non-transient entries should be serializable, otherwise a custom serializer should be
+ * used. Note that putting <code>null</code> value is equivalent to removing the entry for
+ * the given key.
  *
  * @author Lucas Ward
  * @author Douglas Kaminsky