OPENEUROPA-1149: Update the syntax and add documentation block.

src/Commands/DrupalCommands.php

@@ -297,13 +297,27 @@ public function settingsSetup(array $options = [
         $settings_path = $options['root'] . '/sites/' . $options['sites-subdir'] . '/settings.php';
         $settings_override_path = $options['root'] . '/sites/' . $options['sites-subdir'] . '/' . $options['settings-override-file'];
 
+        // Save the filename of the override file in a single variable to use it
+        // in the heredoc variable $custom_config hereunder.
+        $settings_override_filename = $options['settings-override-file'];
+
+        $custom_config = <<< EOF
+/**
+ * Load override configuration, if available.
+ *
+ * This file is generated by the openeuropa/task-runner package.
+ * Use $settings_override_filename to override variables.
+ *
+ * Keep this code block at the end of this file to take full effect.
+ */
+if (file_exists(\$app_root . '/' . \$site_path . '/' . $settings_override_filename)) {
+  include \$app_root . '/' . \$site_path . '/' . $settings_override_filename;
+}
+EOF;
+
         return $this->collectionBuilder()->addTaskList([
             $this->taskFilesystemStack()->copy($settings_default_path, $settings_path, (bool) $options['force']),
-            $this->taskWriteToFile($settings_path)->append()->lines([
-                "if (file_exists(\$app_root . '/' . \$site_path . '/" . $options['settings-override-file'] . "')) {",
-                "  include \$app_root . '/' . \$site_path . '/" . $options['settings-override-file'] . "';",
-                "}"
-            ]),
+            $this->taskWriteToFile($settings_path)->append()->lines([$custom_config]),
             $this->taskWriteConfiguration($settings_override_path, $this->getConfig())->setConfigKey('drupal.settings'),
         ]);
     }