Add example for using @Execution annotation to User Guide (#4525)

Resolves #2669.

---------

Co-authored-by: Marc Philipp <mail@marcphilipp.de>

Author: 2897robo

documentation/src/docs/asciidoc/user-guide/writing-tests.adoc

@@ -3207,6 +3207,17 @@ execution order. Thus, in both cases, test methods in such test classes are only
 concurrently if the `@Execution(CONCURRENT)` annotation is present on the test class or
 method.
 
+You can use the `@Execution` annotation to explicitly configure the execution mode for a
+test class or method:
+
+[source,java]
+----
+include::{testDir}/example/ExplicitExecutionModeDemo.java[]
+----
+
+This allows test classes or methods to opt in or out of concurrent execution regardless of
+the globally configured default.
+
 When parallel execution is enabled and a default `{ClassOrderer}` is registered (see
 <<writing-tests-test-execution-order-classes>> for details), top-level test classes will
 initially be sorted accordingly and scheduled in that order. However, they are not

documentation/src/test/java/example/ExplicitExecutionModeDemo.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package example;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.parallel.Execution;
+import org.junit.jupiter.api.parallel.ExecutionMode;
+
+@Execution(ExecutionMode.CONCURRENT)
+class ExplicitExecutionModeDemo {
+
+	@Test
+	void testA() {
+		// concurrent
+	}
+
+	@Test
+	@Execution(ExecutionMode.SAME_THREAD)
+	void testB() {
+		// overrides to same_thread
+	}
+}