[DOCS] Add security upgrade details (#29100)

Author: lcawl

docs/reference/upgrade/cluster_restart.asciidoc

@@ -128,4 +128,15 @@ GET _cat/recovery
 // CONSOLE
 --
 
-. *Restart machine learning jobs.* 
+. If you use {security} and are upgrading directly to {version} from 5.5 or
+earlier, you must upgrade the `.security` index after you restart {es}.
++
+--
+IMPORTANT: Native realm users cannot authenticate until the index is upgraded.
+For instructions, see {stack-ref}/upgrading-elastic-stack.html#upgrade-internal-indices[Upgrading
+internal indices]. You also need to upgrade the `.security` index if
+you restore a pre-5.6 snapshot to a fresh 6.0 install.
+
+--
+
+. *Restart machine learning jobs.*