Update release notes

Author: mpkorstanje

documentation/src/docs/asciidoc/release-notes/release-notes-6.1.0-M2.adoc

@@ -45,8 +45,8 @@ repository on GitHub.
 [[release-notes-6.1.0-M2-junit-jupiter-new-features-and-improvements]]
 ==== New Features and Improvements
 
-* ❓
-
+* Trim internal stack frames from `AssertionFailedError` stack traces.
+* Introduce new `trimStacktrace(Class<?>)` method for `AssertionFailureBuilder`. It allows user defined assertions to trim their stacktrace.
 
 [[release-notes-6.1.0-M2-junit-vintage]]
 === JUnit Vintage