doc, rpc: mention that listdescriptors result is sorted by string representation

src/wallet/rpc/backup.cpp

@@ -1760,7 +1760,7 @@ RPCHelpMan listdescriptors()
         },
         RPCResult{RPCResult::Type::OBJ, "", "", {
             {RPCResult::Type::STR, "wallet_name", "Name of wallet this operation was performed on"},
-            {RPCResult::Type::ARR, "descriptors", "Array of descriptor objects",
+            {RPCResult::Type::ARR, "descriptors", "Array of descriptor objects (sorted by descriptor string representation)",
             {
                 {RPCResult::Type::OBJ, "", "", {
                     {RPCResult::Type::STR, "desc", "Descriptor string representation"},