roles/nginx: add todo about document roots

We assume it's always /var/www/$domain_name but it can be overriden
in the host_vars...

roles/nginx/tasks/vhosts.yml

@@ -20,6 +20,7 @@
     notify:
       - reload nginx
 
+  # TODO: this could break because we can override the document root in host vars
   - name: Create vhost document roots
     file: path={{ nginx_root_prefix }}/{{ item.domain_name }} state=directory mode=0755 owner=nginx group=nginx
     loop: "{{ nginx_vhosts }}"