Document Failsafe configuration when not using starter parent

See gh-25621

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/docs/asciidoc/using.adoc

@@ -105,6 +105,21 @@ For instance, to use a different version of the SLF4J library and the Spring Dat
 	</dependencyManagement>
 ----
 
+[[failsafe-plugin]]
+=== Using Maven Failsafe Plugin without the Spring Boot Parent POM
+When using the Failsafe plugin, the Spring Boot Parent POM by default configures the `<classesDirectory>` to be `${project.build.outputDirectory}`.
+This configures Failsafe to use the compiled classes rather than the repackaged jar as such:
+
+[source,xml,indent=0]
+----
+<plugin>
+    <groupId>org.apache.maven.plugins</groupId>
+    <artifactId>maven-failsafe-plugin</artifactId>
+    <configuration>
+        <classesDirectory>${project.build.outputDirectory}</classesDirectory>
+    </configuration>
+</plugin>
+----
 
 
 [[using-overriding-command-line]]