Clarify main class finder functionality in Gradle plugin

See gh-1072

spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -451,7 +451,10 @@ The following configuration options are available:
 |`mainClass`
 |The main class that should be run. If not specified the `mainClassName` project property
  will be used or, if the no `mainClassName` id defined the archive will be searched for a
- suitable class.
+ suitable class. "Suitable" means a unique class with a well-formed `main()` method (if
+more than one is found the build will fail). You should also be able to specify the main
+class name via the "run" task (`main` property) and/or the "startScripts" (`mainClassName` 
+property) as an alternative to using the "springBoot" configuration.
 
 |`providedConfiguration`
 |The name of the provided configuration (defaults to `providedRuntime`).