Issue #2978288 by quietone: Improve the documentation on executeMigrations

(cherry picked from commit 6f73a35a)

core/modules/migrate/tests/src/Kernel/MigrateTestBase.php

@@ -184,7 +184,8 @@ protected function executeMigration($migration) {
    * Executes a set of migrations in dependency order.
    *
    * @param string[] $ids
-   *   Array of migration IDs, in any order.
+   *   Array of migration IDs, in any order. If any of these migrations use a
+   *   deriver, the derivatives will be made before execution.
    */
   protected function executeMigrations(array $ids) {
     $manager = $this->container->get('plugin.manager.migration');