docs: Add split_when parameter to split_event processor

_data-prepper/pipelines/configuration/processors/split-event.md

@@ -19,6 +19,7 @@ The following table describes the configuration options for the `split_event` pr
 | `field`          | String  | The event field to be split.                                                           |
 | `delimiter_regex`| String  | The regular expression used as the delimiter for splitting the field.                         |
 | `delimiter`      | String  | The delimiter used for splitting the field. If not specified, the default delimiter is used.  |
+| `split_when`     | String  | A [conditional expression]({{site.url}}{{site.baseurl}}/data-prepper/pipelines/expression-syntax/) that determines whether the processor will be run on the event. If the condition evaluates to `false`, the event passes through unchanged. Default is `null` (all events are processed). |
 
 # Usage
 
@@ -50,3 +51,23 @@ The input will be split into multiple events based on the `query` field, with th
 {"query" : "source", "some_other_field" : "abc" }
 ```
 
+## Conditional splitting with split_when
+
+You can use `split_when` to conditionally apply the split based on event content. This is useful in multi-tenant pipelines where only certain events should be split.
+
+```yaml
+split-event-pipeline:
+  source:
+    http:
+  processor:
+    - split_event:
+        field: body
+        delimiter: "\n"
+        split_when: 'contains(/body, "\n")'
+  sink:
+    - stdout:
+```
+{% include copy.html %}
+
+In this example, the `split_event` processor only splits events where the `body` field contains a newline character. Events without newlines pass through unchanged.
+