Add upgrade tip

Author: pquentin

docs/release-notes/index.md

@@ -21,6 +21,11 @@ To check for security updates, go to [Security announcements for the Elastic sta
 
 ## 9.0.0 (2025-04-15) [elasticsearch-python-client-900-release-notes]
 
+:::{tip}
+Upgrade to Elasticsearch 9 before using elasticsearch-py 9.0.0 or later. Using elasticsearch-py 9.0.0 on an Elasticsearch 8 server will fail.
+Since language clients are forward-compatible, you should first upgrade Elasticsearch, then the Elasticsearch client. See the [compatibility documentation](https://www.elastic.co/docs/reference/elasticsearch/clients/python#_compatibility) for more details.
+:::
+
 ### Breaking changes
 
 * Remove deprecated `Elasticsearch()` options ([#2840](https://github.com/elastic/elasticsearch-py/pull/2840))