ci: add a comment about windows workspace cleanup

ci/Jenkinsfile.windows

@@ -118,6 +118,9 @@ pipeline {
   post {
     success { script { github.notifyPR(true) } }
     failure { script { github.notifyPR(false) } }
+    // Windows workspace often becomes broken if stoped during checkout.
+    // Post cleanup will fail too.
+    // Use 'Wipe out repository and force clone' manual UI option to prevent it.
     cleanup { cleanWs() }
   }
 }