Document application plugin's need for mainClassName to be set

Closes spring-projectsgh-14250

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

@@ -102,7 +102,7 @@ include::../gradle/packaging/spring-boot-dsl-main-class.gradle[tags=main-class]
 ----
 
 If the {application-plugin}[`application` plugin] has been applied its `mainClassName`
-project property can be used for the same purpose:
+project property must be configured and can be used for the same purpose:
 
 [source,groovy,indent=0,subs="verbatim"]
 ----

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

@@ -38,4 +38,6 @@ include::../gradle/publishing/maven-publish.gradle[tags=publishing]
 When the {application-plugin}[`application` plugin] is applied a distribution named
 `boot` is created. This distribution contains the archive produced by the `bootJar` or
 `bootWar` task and scripts to launch it on Unix-like platforms and Windows. Zip and tar
-distributions can be built by the `bootDistZip` and `bootDistTar` tasks respectively.
+distributions can be built by the `bootDistZip` and `bootDistTar` tasks respectively. To
+use the `application` plugin, its `mainClassName` project property must be configured
+with the name of your application's main class.

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

@@ -33,7 +33,7 @@ include::../gradle/running/spring-boot-dsl-main-class-name.gradle[tags=main-clas
 ----
 
 If the {application-plugin}[`application` plugin] has been applied, its `mainClassName`
-project property can be used for the same purpose:
+project property must be configured and can be used for the same purpose:
 
 [source,groovy,indent=0,subs="verbatim"]
 ----