Document target method expectations for ReactiveTransactionManager

Closes gh-23277

spring-tx/src/main/java/org/springframework/transaction/annotation/Transactional.java

@@ -105,6 +105,11 @@ import org.springframework.transaction.TransactionDefinition;
  * all participating data access operations need to execute within the same
  * Reactor context in the same reactive pipeline.
  *
+ * <p><b>Note: When configured with a {@code ReactiveTransactionManager}, all
+ * transaction-demarcated methods are expected to return a reactive pipeline.</b>
+ * Void methods or regular return types need to be associated with a regular
+ * {@code PlatformTransactionManager}, e.g. through {@link #transactionManager()}.
+ *
  * @author Colin Sampaleanu
  * @author Juergen Hoeller
  * @author Sam Brannen

spring-tx/src/main/java/org/springframework/transaction/interceptor/TransactionAspectSupport.java

@@ -358,8 +358,8 @@ public abstract class TransactionAspectSupport implements BeanFactoryAware, Init
 						(isSuspendingFunction ? (hasSuspendingFlowReturnType ? Flux.class : Mono.class) : method.getReturnType());
 				ReactiveAdapter adapter = this.reactiveAdapterRegistry.getAdapter(reactiveType);
 				if (adapter == null) {
-					throw new IllegalStateException("Cannot apply reactive transaction to non-reactive return type: " +
-							method.getReturnType());
+					throw new IllegalStateException("Cannot apply reactive transaction to non-reactive return type [" +
+							method.getReturnType() + "] with specified transaction manager: " + tm);
 				}
 				return new ReactiveTransactionSupport(adapter);
 			});