Update documentation about serializable keys in the execution context

Resolves #4457

(cherry picked from commit dcd1ac8)

Author: hwan33

spring-batch-docs/asciidoc/domain.adoc

@@ -556,6 +556,12 @@ 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` is the persistence mechanism for all of the Stereotypes mentioned above.

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2018 the original author or authors.
+ * Copyright 2006-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,8 +29,10 @@
  * for a map that allows optionally for type safety on reads. It also allows for
  * dirty checking by setting a 'dirty' flag whenever any put is called.
  *
- * 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