Make classloader customization easier to discover

If you encounter the problem of the two classloaders in Devtools, there's no hint that there are two classloaders and that some configuration changes might help you. You only get an IllegalArgumentException. Googling does not help because the exception is not mentioned in the docs. Futhermore, the docs sound like the problem can only occur in IDEs, which is not the case.

Author: aahlenst

spring-boot-project/spring-boot-docs/src/docs/asciidoc/using/devtools.adoc

@@ -217,10 +217,18 @@ For IntelliJ IDEA, you can follow the https://www.jetbrains.com/help/idea/spring
 As described earlier in the <<using#using.devtools.restart.restart-vs-reload>> section, restart functionality is implemented by using two classloaders.
 For most applications, this approach works well.
 However, it can sometimes cause classloading issues.
+These might manifest in an exception like:
+
+[source,text]
+----
+Caused by: java.lang.IllegalArgumentException: non-public interface is not defined by the given loader
+----
 
 By default, any open project in your IDE is loaded with the "`restart`" classloader, and any regular `.jar` file is loaded with the "`base`" classloader.
 If you work on a multi-module project, and not every module is imported into your IDE, you may need to customize things.
-To do so, you can create a `META-INF/spring-devtools.properties` file.
+The same is true if you use `mvn spring-boot:run` or `gradle bootRun` in a multi-module project: The project containing your `@SpringBootApplication` is loaded with the "`restart`" classloader, and everything else with the "`base`" classloader.
+
+To instruct Spring Boot to load `.jar` files with a different classloader, you can create a `META-INF/spring-devtools.properties` file.
 
 The `spring-devtools.properties` file can contain properties prefixed with `restart.exclude` and `restart.include`.
 The `include` elements are items that should be pulled up into the "`restart`" classloader, and the `exclude` elements are items that should be pushed down into the "`base`" classloader.