docs: note bitcoind peer-check RPC switch

Document that bitcoind outbound peer health checks now use
getnetworkinfo.connections_out instead of getpeerinfo.

Also mention that texts of zmq port mismatch warnings were fixed.

Author: starius

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

@@ -70,6 +70,11 @@
   the chain watch filter on restart. This was a pre-existing bug since
   private taproot channels were first introduced.
 
+* Bitcoind outbound peer health checks [now use](https://github.com/lightningnetwork/lnd/pull/10686)
+  `getnetworkinfo.connections_out` instead of `getpeerinfo`. The same PR also
+  [clarifies](https://github.com/lightningnetwork/lnd/issues/10568) the ZMQ
+  port-mismatch warnings so they no longer suggest that the connection failed.
+
 # New Features
 
 - [Basic Support](https://github.com/lightningnetwork/lnd/pull/9868) for onion