Add polygon.sync notes to changelog (#13451)

ChangeLog.md

@@ -8,6 +8,10 @@ ChangeLog
 - `--prune.mode` default is `full`. For compatibility with `geth` and other clients. Plz set explicit
   `--prune.mode` flag to your Erigon3 setups - to simplify future upgrade/downgrade.
 - Bor chains: enable our internal Consensus Layer by default (name: Astrid)
+    - The process should auto upgrade - in which case you may find that it starts creating new snapshots for checkpoints and milestones.
+    - This may however fail, as there are a number of potential edge cases.  If this happens the process will likely stop with a failure message.
+    - In this situation you will need to do a clean sync, in which case the complete snapshot set will be downloaded and astrid will sync.
+    - If you want to prevent this and retain the old behaviour start erigon with --polygon.sync=false
 - Released binaries for Linux are now dynamically linked (
   Issues: https://github.com/erigontech/erigon/issues/12570, https://github.com/erigontech/erigon/issues/12556 )