Update release notes

docs/guides/admin/docs/releasenotes.md

@@ -2,6 +2,12 @@
 
 ## Opencast 15.6
 
+**Attention:**
+The fix for the handling of captions by the select-tracks operation
+([5829](https://github.com/opencast/opencast/pull/5829)) requires the removal of the operation directly afterwards from
+the `partial-publish` community workflow, otherwise captions will be duplicated during publication (see
+[6046](https://github.com/opencast/opencast/pull/6046) which is _not_ included in this release).
+
 ### Behavior changes
 
 - **Handle lang tag by asset upload correctly:**