docs: add v0.20.4 invoice migration note

Author: ziggie1984

docs/release-notes/release-notes-0.20.4.md

@@ -39,6 +39,12 @@
   incoming HTLC resolver could treat a foreign commitment spend as its own
   success transaction and offer a phantom input to the sweeper.
 
+* Native SQL invoice migration [now correctly associates legacy AMP invoice
+  HTLCs](https://github.com/lightningnetwork/lnd/pull/11106) with their AMP
+  sub-invoices. Previously, the HTLC rows were inserted without those
+  associations, causing verification to fail and the migration transaction to
+  roll back.
+
 # New Features
 
 ## Functional Enhancements