Add modular Javadoc to 5.6.0 release notes

Author: juliette-derancourt

documentation/src/docs/asciidoc/release-notes/release-notes-5.6.0.adoc

@@ -31,6 +31,7 @@ milestone pages in the JUnit repository on GitHub.
 * https://docs.gradle.org/6.0-rc-1/userguide/publishing_gradle_module_metadata.html[Gradle
   Module Metadata] is now published for all artifacts.
 * OSGi metadata is now published in all binary JARs.
+* Javadoc now contains a module API overview page.
 
 
 [[release-notes-5.6.0-junit-platform]]