Reference the docs for ES/OS refilling in changelog

changelog.d/0-release-notes/elasticsearch-to-opensearch

@@ -4,5 +4,7 @@ not compatible. I.e., either a migration is required
 (https://opensearch.org/docs/latest/upgrade-to/) or the indexes need to be
 rebuilt from scratch. The latter is done automatically when the
 `elasticsearch-index-create` job of the `elasticsearch-index` Helm chart faces
-an OpenSearch instance where Wire's indexes are missing. Depending on the amount
-of users in the database, this may take long.
+an OpenSearch instance where Wire's indexes are missing. Alternatively, this can
+be started manually; see
+https://docs.wire.com/developer/reference/elastic-search.html#refill-es-documents-from-cassandra
+. Depending on the amount of users in the database, this may take long.