docs: add release not for network separation safeguard

This is in particular important when running with a postgres
backend.

This only works if you run LND with the native sql flag but
people should run it with this flag from 21 on anyways.

Author: ziggie1984

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

@@ -257,7 +257,15 @@
 
 ## Database
 
-* Freeze the [graph SQL migration 
+* [Prevent silent data corruption](https://github.com/lightningnetwork/lnd/pull/10354)
+  when reusing the same database across different Bitcoin networks. On first
+  startup the active network is persisted in a new `metadata` table; on every
+  subsequent restart lnd compares the stored value against the configured
+  network and refuses to start if they differ, printing a clear error message
+  with remediation steps. This safeguard applies to both the PostgreSQL and
+  SQLite backends.
+
+* Freeze the [graph SQL migration
   code](https://github.com/lightningnetwork/lnd/pull/10338) to prevent the 
   need for maintenance as the sqlc code evolves. 
 * Prepare the graph DB for handling gossip V2