Document that @Transactional does not propagate to new threads

Closes gh-25439

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

@@ -38,16 +38,25 @@ import org.springframework.transaction.TransactionDefinition;
  * {@link org.springframework.transaction.interceptor.RuleBasedTransactionAttribute}
  * class, and in fact {@link AnnotationTransactionAttributeSource} will directly
  * convert the data to the latter class, so that Spring's transaction support code
- * does not have to know about annotations. If no rules are relevant to the exception,
- * it will be treated like
- * {@link org.springframework.transaction.interceptor.DefaultTransactionAttribute}
- * (rolling back on {@link RuntimeException} and {@link Error} but not on checked
- * exceptions).
+ * does not have to know about annotations. If no custom rollback rules apply,
+ * the transaction will roll back on {@link RuntimeException} and {@link Error}
+ * but not on checked exceptions.
  *
  * <p>For specific information about the semantics of this annotation's attributes,
  * consult the {@link org.springframework.transaction.TransactionDefinition} and
  * {@link org.springframework.transaction.interceptor.TransactionAttribute} javadocs.
  *
+ * <p>This annotation commonly works with thread-bound transactions managed by
+ * {@link org.springframework.transaction.PlatformTransactionManager}, exposing a
+ * transaction to all data access operations within the current execution thread.
+ * <b>Note: This does NOT propagate to newly started threads within the method.</b>
+ *
+ * <p>Alternatively, this annotation may demarcate a reactive transaction managed
+ * by {@link org.springframework.transaction.ReactiveTransactionManager} which
+ * 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.
+ *
  * @author Colin Sampaleanu
  * @author Juergen Hoeller
  * @author Sam Brannen

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

@@ -28,7 +28,7 @@ import org.springframework.lang.Nullable;
 /**
  * TransactionAttribute implementation that works out whether a given exception
  * should cause transaction rollback by applying a number of rollback rules,
- * both positive and negative. If no rules are relevant to the exception, it
+ * both positive and negative. If no custom rollback rules apply, this attribute
  * behaves like DefaultTransactionAttribute (rolling back on runtime exceptions).
  *
  * <p>{@link TransactionAttributeEditor} creates objects of this class.