Document virtual threads limitations

This commit adds a new section in the Spring Boot reference
documentation to mention potential throughput limitations with Java
virtual threads support.

This section links to the official Java documentation which expands much
more on this matter.

Closes gh-38883

Author: bclozel

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/spring-application.adoc

@@ -384,10 +384,14 @@ Spring Boot can also be configured to expose a {spring-boot-actuator-restapi-doc
 === Virtual threads
 If you're running on Java 21 or up, you can enable virtual threads by setting the property configprop:spring.threads.virtual.enabled[] to `true`.
 
+Before turning on this option for your application, you should consider https://docs.oracle.com/en/java/javase/21/core/virtual-threads.html[reading the official Java virtual threads documentation].
+In some cases, applications can experience lower throughput because of "Pinned Virtual Threads"; this page also explains how to detect such cases with JDK Flight Recorder or the `jcmd` CLI.
+
 WARNING: One side effect of virtual threads is that these threads are daemon threads.
 A JVM will exit if there are no non-daemon threads.
 This behavior can be a problem when you rely on, e.g. `@Scheduled` beans to keep your application alive.
 If you use virtual threads, the scheduler thread is a virtual thread and therefore a daemon thread and won't keep the JVM alive.
 This does not only affect scheduling, but can be the case with other technologies, too!
 To keep the JVM running in all cases, it is recommended to set the property configprop:spring.main.keep-alive[] to `true`.
 This ensures that the JVM is kept alive, even if all threads are virtual threads.
+