Document the /dev/null option in the class doc block

src/Plugin/migrate/source/CSV.php

@@ -12,7 +12,8 @@ use League\Csv\Reader;
  * Source for CSV files.
  *
  * Available configuration options:
- * - path: Path to the  CSV file. File streams are supported.
+ * - path: Path to the  CSV file. File streams are supported. Use /dev/null to
+ *   get an empty source (useful for migration_lookup).
  * - ids: Array of column names that uniquely identify each record. Column
  *   names used as IDs can only contain letters (uppercase and lowercase),
  *   numbers (0-9), and underscores. No spaces or other special characters