Document target method expectations for ReactiveTransactionManager

Closes gh-23277

framework-docs/modules/ROOT/pages/data-access/transaction/declarative/tx-decl-explained.adoc

@@ -38,6 +38,11 @@ within the method.
 A reactive transaction managed by `ReactiveTransactionManager` uses the Reactor context
 instead of thread-local attributes. As a consequence, all participating data access
 operations need to execute within the same Reactor context in the same reactive pipeline.
+
+When configured with a `ReactiveTransactionManager`, all transaction-demarcated methods
+are expected to return a reactive pipeline. Void methods or regular return types need
+to be associated with a regular `PlatformTransactionManager`, e.g. through the
+`transactionManager` attribute of the corresponding `@Transactional` declarations.
 ====
 
 The following image shows a conceptual view of calling a method on a transactional proxy:

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

@@ -105,6 +105,11 @@
  * 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 @@ protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targe
 						(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);
 			});