Add note about Kotlin main class name

See gh-23418

Author: Sebastiaan Fernandez

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/packaging.adoc

@@ -154,6 +154,11 @@ include::../gradle/packaging/boot-jar-manifest-main-class.gradle[tags=main-class
 include::../gradle/packaging/boot-jar-manifest-main-class.gradle.kts[tags=main-class]
 ----
 
+NOTE: Although the use of Kotlin is demonstrated in some examples above, a main class written in Java is assumed.
+If however the main class is written in Kotlin, the name of the generated Java class should be used.
+By default, this is the name of the Kotlin class with the `Kt` suffix added.
+For example: `ExampleApplication` becomes `ExampleApplicationKt`.
+(If another name is defined using the `@JvmName` annotation, then that name should be used.)
 
 [[packaging-executable-configuring-excluding-devtools]]
 ==== Excluding Devtools