Document how TestExecutionListener are called (#3092)

Resolves #3061.

Author: juliette-derancourt

documentation/src/docs/asciidoc/user-guide/advanced-topics/launcher-api.adoc

@@ -71,6 +71,11 @@ There is no return value for the `execute()` method, but you can use a
 `{SummaryGeneratingListener}`, `{LegacyXmlReportGeneratingListener}`, and
 `{UniqueIdTrackingListener}`.
 
+NOTE: All `TestExecutionListener` methods are called sequentially. Methods for start
+events are called in registration order while methods for finish events are called in
+reverse order.
+Test case execution won't start before all `executionStarted` calls have returned.
+
 [[launcher-api-engines-custom]]
 ==== Registering a TestEngine
 

junit-platform-launcher/src/main/java/org/junit/platform/launcher/TestExecutionListener.java

@@ -25,6 +25,11 @@
  * Concrete implementations may therefore override one or more of these methods
  * to be notified of the selected events.
  *
+ * <p>All {@code TestExecutionListener} methods are called sequentially. Methods
+ * for start events are called in registration order while methods for finish
+ * events are called in reverse order. Test case execution won't start before
+ * all {@link #executionStarted(TestIdentifier)} calls have returned.
+ *
  * <p>JUnit provides two example implementations.
  *
  * <ul>