Resolve #3511080 "Officially define our"

Closes #3511080

src/Activator/ActivationStatus.php

@@ -6,6 +6,10 @@ namespace Drupal\project_browser\Activator;
 
 /**
  * Defines the possible states of a project in the current site.
+ *
+ * @api
+ *   This enum is covered by our backwards compatibility promise and can be
+ *   safely relied upon.
  */
 enum ActivationStatus {