Document __parent__ namespace for Twig

docs/simplesamlphp-theming.md

@@ -131,7 +131,7 @@ If you need to make more extensive customizations to the base template, you shou
 
 Any references to `$this->data['baseurlpath']` in old-style templates can be replaced with `{{baseurlpath}}` in Twig templates. Likewise, references to `\SimpleSAML\Module::getModuleURL()` can be replaced with `{{baseurlpath}}module.php/mymodule/...`
 
-Within templates each module is defined as a separate namespace matching the module name. This allows one template to reference templates from other modules using Twig's `@namespace_name/template_path` notation. For instance, a template in `mymodule` can include the widget template from the `yourmodule` module using the notation `@yourmodule/widget.twig`.
+Within templates each module is defined as a separate namespace matching the module name. This allows one template to reference templates from other modules using Twig's `@namespace_name/template_path` notation. For instance, a template in `mymodule` can include the widget template from the `yourmodule` module using the notation `@yourmodule/widget.twig`. A special namespace, `__parent__`, exists to allow theme developers to more easily extend a module's stock template.
 
 Even more advanced changes can be made by defining a theme controller in `config.php`: