Issue #3285631: Make it impossible to call CronUpdater::begin() directly

src/CronUpdater.php

@@ -97,6 +97,13 @@ class CronUpdater extends Updater {
    * {@inheritdoc}
    */
   final public function begin(array $project_versions, ?int $timeout = 300): string {
+    // Unattended updates should never be started using this method. They should
+    // only be done by ::handleCron(), which has a strong opinion about which
+    // release to update to. Throwing an exception here is just to enforce this
+    // boundary. To update to a specific version of core, use
+    // \Drupal\automatic_updates\Updater::begin() (which is called in
+    // ::performUpdate() to start the update to the target version of core
+    // chosen by ::handleCron()).
     throw new \BadMethodCallException(__METHOD__ . '() cannot be called directly.');
   }
 
@@ -120,6 +127,7 @@ class CronUpdater extends Updater {
     // handle any exceptions or validation errors consistently, and destroy the
     // stage regardless of whether the update succeeds.
     try {
+      // @see ::begin()
       parent::begin(['drupal' => $target_version], $timeout);
       $this->stage();
       $this->apply();