Clarifying nested $routes->group() options [ci skip]

[tangix]

Description
Added to the documentation that when nesting $routes->group(), the options passed to the outer group() is not merged with the inner group(). A definition such as:

$routes->group('config', ['filter' => 'myfilter:config'], function ($routes) {
    $routes->get('/', 'Home::index');
    $routes->group('region', ['filter' => 'myfilter:region'], function ($routes) {
        $routes->get('/', 'Home::index');
    });
});

would only call myfilter:region for config/region which may not be what a user expects.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide