Update documentation on how to configure the main class with Gradle

In 1.3, our Gradle plugin no longer automatically applies the
application plugin. This affects the default options that are
available for explicitly configuring a project's main class. This
commit updates the documentation accordingly.

Closes gh-3768

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

@@ -366,12 +366,14 @@ The following configuration options are available:
 want the other Boot features but not this one)
 
 |`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" 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.
+|The main class that should be run. If not specified, and you have applied the application
+ plugin, the `mainClassName` project property will be used. If the application plugin has
+ not been applied or no `mainClassName` has been specified, the archive will be searched
+ for a suitable class. "Suitable" means a unique class with a well-formed `main()` method
+ (if more than one is found the build will fail). If you have applied the application
+ plugin, the main class can also be specified via its "run" task (`main` property) and/or
+ its "startScripts" task (`mainClassName` property) as an alternative to using the
+ "springBoot" configuration.
 
 |`classifier`
 |A file name segment (before the extension) to add to the archive, so that the original is