doc: release note for wallet RPCs "warning" field deprecation

doc/release-notes-27279.md

@@ -0,0 +1,10 @@
+Wallet
+------
+
+- In the createwallet, loadwallet, unloadwallet, and restorewallet RPCs, the
+  "warning" string field is deprecated in favor of a "warnings" field that
+  returns a JSON array of strings to better handle multiple warning messages and
+  for consistency with other wallet RPCs. The "warning" field will be fully
+  removed from these RPCs in v26. It can be temporarily re-enabled during the
+  deprecation period by launching bitcoind with the configuration option
+  `-deprecatedrpc=walletwarningfield`. (#27279)