Add support for Coroutines transactions

This commit adds Coroutines extensions for
TransactionalOperator.transactional that accept suspending lambda or
Kotlin Flow parameters.

@Transactional on suspending functions is not supported yet, gh-23575
has been created for that purpose.

Closes gh-22915

src/docs/asciidoc/languages/kotlin.adoc

@@ -574,6 +574,12 @@ class UserHandler(builder: WebClient.Builder) {
 }
 ----
 
+=== Transactions
+
+Transactions on Coroutines are supported via the programmatic variant of the Reactive
+transaction management provided as of Spring Framework 5.2. `TransactionalOperator.transactional`
+extensions with suspending lambda and Kotlin `Flow` parameter are provided for that purpose.
+
 [[kotlin-spring-projects-in-kotlin]]
 == Spring Projects in Kotlin