Issue #2168153 by chx: Add warning to Variable class that it is not a good...

Issue #2168153 by chx: Add warning to Variable class that it is not a good class to use as a source class example

core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/source/d6/Variable.php

@@ -12,6 +12,9 @@
 /**
  * Drupal 6 variable source from database.
  *
+ * This source class always returns a single row and as such is not a good
+ * example for any normal source class returning multiple rows.
+ *
  * @PluginID("drupal6_variable")
  */
 class Variable extends Drupal6SqlBase {