wallet: document structured importdescriptors errors

Replace the elided "JSONRPC error" placeholder in the
importdescriptors RPC help with explicit "code" and "message"
fields.

This makes the documented result schema match the structured
error object returned in practice.

src/wallet/rpc/backup.cpp

@@ -343,7 +343,8 @@ RPCHelpMan importdescriptors()
                             }},
                             {RPCResult::Type::OBJ, "error", /*optional=*/true, "",
                             {
-                                {RPCResult::Type::ELISION, "", "JSONRPC error"},
+                                {RPCResult::Type::NUM, "code", "JSONRPC error code"},
+                                {RPCResult::Type::STR, "message", "JSONRPC error message"},
                             }},
                         }},
                     }