Fix Kdoc regression to show only Kotlin API

As of Gradle Kotlin plugin 1.1.60,
project.sourceSets.main.kotlin.srcDirs contains
Java source directories in addition to Kotlin
ones. This commit filter out Java sources to
retain only Kotlin ones.

Issue: SPR-16268

gradle/docs.gradle

@@ -66,7 +66,8 @@ dokka {
 	outputDirectory = "$buildDir/docs/kdoc"
 
 	sourceDirs = files(subprojects.collect { project ->
-		project.sourceSets.main.kotlin.srcDirs
+		def kotlinDirs = project.sourceSets.main.kotlin.srcDirs.collect()
+		kotlinDirs -= project.sourceSets.main.java.srcDirs
 	})
 	externalDocumentationLink {
 		url = new URL("http://docs.spring.io/spring-framework/docs/${version}/javadoc-api/")