Add documentation for SimpleAsyncTaskExecutorBuilder

See gh-35711

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/task-execution-and-scheduling.adoc

@@ -49,4 +49,5 @@ The thread pool uses one thread by default and its settings can be fine-tuned us
 	        size: 2
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-Both a `ThreadPoolTaskExecutorBuilder` bean and a `TaskSchedulerBuilder` bean are made available in the context if a custom executor or scheduler needs to be created.
+A `ThreadPoolTaskExecutorBuilder` bean, a `SimpleAsyncTaskExecutorBuilder` bean and a `TaskSchedulerBuilder` bean are made available in the context if a custom executor or scheduler needs to be created.
+The `SimpleAsyncTaskExecutorBuilder` is auto-configured to use virtual threads if they are enabled (using Java 21+ and configprop:spring.threads.virtual.enabled[] set to `true`).