Updated UPGRADING.rst for new Elasticsearch behavior: retry connecting on startup (Graylog2#10795)

* Updated UPGRADING.rst for new Elasticsearch behavior

* linked to docs

Author: janheise

UPGRADING.rst

@@ -13,6 +13,19 @@ Upgrading to Graylog 4.1.x
 Breaking Changes
 ================
 
+
+Changes to the Elasticsearch Support
+------------------------------------
+
+When you have version-probing for the used Elasticsearch version enabled, and Graylog starts up but can not
+connect to ES, the startup stopped immediately with v4.0 and prior. Starting from 4.1 the default behaviour is,
+that Graylog retries connecting with a delay until it can connect to Elasticsearch. See the Elasticsearch
+configuration_ for details.
+
+.. _configuration: https://docs.graylog.org/en/4.1/pages/configuration/elasticsearch.html
+
+Configuration options: ``elasticsearch_version_probe_attempts`` and ``elasticsearch_version_probe_delay``.
+
 Configuration file changes
 --------------------------