Document namespaces (cf https://github.com/simplesamlphp/simplesamlphp/wiki/Twig-conventions)

docs/simplesamlphp-theming.md

@@ -131,6 +131,8 @@ 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`.
+
 Even more advanced changes can be made by defining a theme controller in `config.php`:
 
     'theme.controller' => '\SimpleSAML\Module\mymodule\FancyThemeController',