Add a known issue for upgrading from 5.x to 6.3.0 (#31501)

This is due to #31482

Author: bleskes

docs/reference/release-notes/6.3.asciidoc

@@ -1,6 +1,17 @@
 [[release-notes-6.3.0]]
 == {es} version 6.3.0
 
+[float]
+=== Known issues
+
+Upgrades from any 5.x version will fail for indexes which are prepared using the <<indices-synced-flush, _synced flush API>>,
+or were automatically sync-flushed due to inactivity. If upgrading from those versions, please
+wait for 6.3.1 to be released.
+
+Clusters with a Gold or Platinum license that are upgrading to 6.3 will need to explicitly set
+`xpack.security.enabled: true` in `elasticsearch.yml` to upgrade successfully.
+If this value is not set, the cluster will be unable to form after upgrade.
+
 Also see <<breaking-changes-6.3>>.
 
 [[breaking-6.3.0]]