Add a comment to setBreadcrumbs

Author: romtsn

sentry/src/main/java/io/sentry/cache/PersistingScopeObserver.java

@@ -136,6 +136,8 @@ public void addBreadcrumb(@NotNull Breadcrumb crumb) {
   @Override
   public void setBreadcrumbs(@NotNull Collection<Breadcrumb> breadcrumbs) {
     if (breadcrumbs.isEmpty()) {
+      // we only clear the queue if the new collection is empty (someone called clearBreadcrumbs)
+      // If it's not empty, we'd add breadcrumbs one-by-one in the method above
       serializeToDisk(
           () -> {
             try {