Polish RecordApplicationEvents javadoc: add note on captured threads

This commit clarifies that the annotation is used to capture events that
are fired from the test `Thread` or descendant threads, since the test
framework uses `InheritableThreadLocal` to store captured events now.

See gh-31079
See gh-30020

spring-test/src/main/java/org/springframework/test/context/event/RecordApplicationEvents.java

@@ -28,7 +28,8 @@
  * instruct the <em>Spring TestContext Framework</em> to record all
  * {@linkplain org.springframework.context.ApplicationEvent application events}
  * that are published in the {@link org.springframework.context.ApplicationContext
- * ApplicationContext} during the execution of a single test.
+ * ApplicationContext} during the execution of a single test, either from the
+ * test thread or its descendants.
  *
  * <p>The recorded events can be accessed via the {@link ApplicationEvents} API
  * within your tests.