Added short section about ClassRule for JenkinsRule (#7756)

added short section about ClassRule for JenkinsRule

content/doc/developer/testing/index.adoc

@@ -649,4 +649,11 @@ TODO: RestartableJenkinsRule.
 === Custom builder
 
 == Advanced and Tips etc.
-//Tip: Use @ClassRule for JenkinsRule, if you're 900% sure that everything is ok
+
+This section covers advanced topics and tips to improve testing.
+
+=== `ClassRule` for `JenkinsRule`
+
+Use `@ClassRule` with `JenkinsRule` when you want to share a single Jenkins instance across all tests in the class, especially when you need to avoid repeated setup and teardown of the Jenkins environment.
+It’s a great way to optimize your tests when you don’t need a fresh Jenkins instance for every single test method, but instead want to run tests that all rely on the same shared Jenkins setup.
+The field annotated with `@ClassRule` must be static because it is shared across all instances of the test class.