docs: add release note for two-phase channel close

Documents the two-phase channel close change (PR #10732) in the 0.21.0
release notes under the Database section.

Author: ziggie1984

docs/release-notes/release-notes-0.21.0.md

@@ -369,6 +369,17 @@
     `native-sql` setting enabled.
 
 
+* [Channel close is now split into two phases to avoid long write-lock
+  hold times on the KV-over-SQL
+  backends](https://github.com/lightningnetwork/lnd/pull/10732). Phase 1
+  atomically removes the channel from all open-channel views in a small
+  O(1) transaction. Phase 2 deletes the bulk historical data (revocation
+  log entries, forwarding packages) in small batches of
+  `DefaultPurgeBatchSize` (500) entries per transaction, keeping each
+  write-lock window short. Pending Phase 2 cleanups are persisted and
+  automatically resumed on node restart, so no data is left dangling
+  after an interrupted shutdown.
+
 ## Code Health
 
 * [Update taproot detection](https://github.com/lightningnetwork/lnd/pull/10683)