docs: add precisions to multi container compose file (#2650)

Co-authored-by: Nicolas Duchon <nicolas.duchon@gmail.com>

docker-compose-separate-containers.yml

@@ -9,10 +9,14 @@ services:
       - "80:80"
     volumes:
       - nginx_conf:/etc/nginx/conf.d:ro
+      - network_internal.conf:/etc/nginx/network_internal.conf:ro
 
   dockergen:
     image: nginxproxy/docker-gen
     command: -notify-sighup nginx -watch /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
+    # the name "nginx" passed to the "-notify-sighup" flag must exactly match the container name used for the nginx container above.
+    environment:
+      # nginx-proxy environment variable (HTTP_PORT etc.) must be set on the docker-gen container (not the nginx container)  
     volumes:
       - /var/run/docker.sock:/tmp/docker.sock:ro
       - ./nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl