Document exception to statistics.refresh! & workaround

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -317,7 +317,18 @@ the displayed size may still show old sizes or commit numbers. To force an updat
 p = Project.find_by_full_path('<namespace>/<project>')
 pp p.statistics
 p.statistics.refresh!
-pp p.statistics  # compare with earlier values
+pp p.statistics
+# compare with earlier values
+
+# check the total artifact storage space separately
+builds_with_artifacts = p.builds.with_downloadable_artifacts.all
+
+artifact_storage = 0
+builds_with_artifacts.find_each do |build|
+  artifact_storage += build.artifacts_size
+end
+
+puts "#{artifact_storage} bytes"
 ```
 
 ### Identify deploy keys associated with blocked and non-member users