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You can use it that way :

function add_suffix_to_title($title, $id = null){
    if (! is_admin()) {
        return '<h1>'.$title.'</h1> Your Suffix';
    }
    return $title;
}

add_action( 'the_title', 'add_suffix_to_title', 10, 1 );

And when you invoke the "the_title" function remove the h1 tag :

the_title();

I hope that would help you.