[Backport 8.x] Update changelog for 8.15.1 (#2387)

Co-authored-by: Josh Mock <[email protected]>

Author: github-actions[bot]

docs/changelog.asciidoc

@@ -1,6 +1,17 @@
 [[changelog-client]]
 == Release notes
 
+[discrete]
+=== 8.15.1
+
+[discrete]
+==== Features
+
+[discrete]
+===== Improved support for Elasticsearch `v8.15`
+
+Updated TypeScript types based on fixes and improvements to the Elasticsearch specification.
+
 [discrete]
 === 8.15.0