[Docs] Reverse proxy article: nginx config fix

docs/reverse-proxy.rst

@@ -22,20 +22,25 @@ The following block represents the minimal `nginx` configuration:
 If you run Flower behind custom location, make sure :ref:`url_prefix` option
 value equals to the location path.
 
-For instance, for `url_prefix` = **flower** (or **/flower**) you need the following
-`nginx` configuration:
+You have to use either environment variable `FLOWER_URL_PREFIX=flower`
+or command parameter `--url_prefix=flower` when you run it
+via `celery`. With that being set you need the following `nginx` configuration:
 
 .. code-block:: nginx
 
     server {
         listen 80;
-        server_name flower.example.com;
+        server_name example.com;
 
         location /flower/ {
-            proxy_pass http://localhost:5555;
+            proxy_pass http://localhost:5555/flower/;
         }
     }
 
+without `url_prefix` Flower frontend won't be able to generate
+correct static links, and without `/flower/` at the end of `proxy_pass`
+parameter, the browser will lead you to 404.
+
 Note that you should not expose this site to the public internet without
 any sort of authentication! If you have a `htpasswd` file with user
 credentials you can make `nginx` use this file by adding the following