Document #4689 in release notes

Author: marcphilipp

documentation/src/docs/asciidoc/release-notes/release-notes-5.13.3.adoc

@@ -10,6 +10,16 @@ link:{junit-framework-repo}+/milestone/100?closed=1+[5.13.3] milestone page in t
 repository on GitHub.
 
 
+[[release-notes-5.13.3-overall-improvements]]
+=== Overall Changes
+
+[[release-notes-5.13.3-overall-new-features-and-improvements]]
+==== New Features and Improvements
+
+* All _experimental_ APIs have been promoted to _maintained_ to indicate that they won't
+  be removed in any future 5.x release.
+
+
 [[release-notes-5.13.3-junit-platform]]
 === JUnit Platform