docs: marked `foreach` processor as experimental

Closes #19602

docs/reference/ingest/ingest-node.asciidoc

@@ -932,6 +932,12 @@ to the requester.
 
 [[foreach-processor]]
 === Foreach Processor
+
+experimental[This processor may change or be replaced by something else that provides similar functionality. This
+processor executes in its own context, which makes it different compared to all other processors and for features like
+verbose simulation the subprocessor isn't visible. The reason we still expose this processor, is that it is the only
+processor that can operate on an array]
+
 Processes elements in an array of unknown length.
 
 All processors can operate on elements inside an array, but if all elements of an array need to

modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/ForEachProcessor.java

@@ -38,6 +38,8 @@ import static org.elasticsearch.ingest.ConfigurationUtils.readStringProperty;
  *
  * This can be useful in cases to do string operations on json array of strings,
  * or remove a field from objects inside a json array.
+ *
+ * Note that this processor is experimental.
  */
 public final class ForEachProcessor extends AbstractProcessor {