Fix Dokka reference to Spring Framework's Javadoc

This commit specifies a local packageListUrl and defines that dokka task
must be executed after the api task in order to be able to build KDoc
during the release process when the Spring Framework's Javadoc is not
published yet.

Issue: SPR-16687

gradle/docs.gradle

@@ -49,12 +49,9 @@ task api(type: Javadoc) {
 	}
 }
 
-// Need https://github.com/Kotlin/dokka/issues/184 to be fixed to avoid "Can't find node by signature" log spam
 dokka {
 	dependsOn {
-		subprojects.collect {
-			it.tasks.getByName("jar")
-		}
+		tasks.getByName("api")
 	}
 	doFirst {
 		classpath = subprojects.collect { project -> project.jar.outputs.files.getFiles() }.flatten()
@@ -69,6 +66,10 @@ dokka {
 		def kotlinDirs = project.sourceSets.main.kotlin.srcDirs.collect()
 		kotlinDirs -= project.sourceSets.main.java.srcDirs
 	})
+	externalDocumentationLink {
+		url = new URL("https://docs.spring.io/spring-framework/docs/$version/javadoc-api/")
+		packageListUrl = new File(buildDir, "api/package-list").toURI().toURL()
+	}
 	externalDocumentationLink {
 		url = new URL("http://projectreactor.io/docs/core/release/api/")
 	}