doc: add release note for include_watchonly default changes

Signed-off-by: William Casarin <jb55@jb55.com>

doc/release-notes-16383.md

@@ -0,0 +1,8 @@
+RPC changes
+-----------
+
+RPCs which have an `include_watchonly` argument or `includeWatching`
+option now default to `true` for watch-only wallets. Affected RPCs
+are: `getbalance`, `listreceivedbyaddress`, `listreceivedbylabel`,
+`listtransactions`, `listsinceblock`, `gettransaction`,
+`walletcreatefundedpsbt`, and `fundrawtransaction`.