Update serverless-changelog.asciidoc (#2997)

Author: jmikell821

docs/en/install-upgrade/serverless-changelog.asciidoc

@@ -29,6 +29,7 @@ For serverless changes in Cloud Console, refer to https://www.elastic.co/guide/e
 * Adds support for the MV_EXPAND command with the ES|QL rule type ({kibana-pull}212675[#212675]).
 * Enables endpoint actions for events ({kibana-pull}206857[#206857]).
 * The {ref}/semantic-text.html[`semantic_text`] field type is now GA on {{serverless-full}}.
+* Adds the ability for users to https://github.com/elastic/kibana/issues/174168[customize prebuilt rules]. Users can modify most rule parameters, export and import prebuilt rules — including customized ones — and upgrade prebuilt rules while retaining customization settings ({kibana-pull}212761[#212761]).
 
 [discrete]
 [[fixes-03172025]]