Update docs nginx reverse proxy example (#2443)

* Update docs nginx reverse proxy example

According to #817 P3P header is mandatory. Otherwise django CSRF breaks login

* Note P3P header optional

Co-authored-by: shamoon <4887959+shamoon@users.noreply.github.com>

docs/setup.md

@@ -881,6 +881,7 @@ http {
             proxy_set_header X-Real-IP $remote_addr;
             proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
             proxy_set_header X-Forwarded-Host $server_name;
+            add_header P3P 'CP=""'; # may not be required in all setups
         }
     }
 }