Update avoid-php-fpm-reloading.md (#4027)

I've also been setting `DOCUMENT_ROOT` to also include the fix. I'm sure `SCRIPT_FILENAME` is enough in most cases, but also setting `DOCUMENT_ROOT` would ensure the expectation in all scenarios.

Author: adrum

docs/avoid-php-fpm-reloading.md

@@ -27,14 +27,16 @@ _current_ in the path, your server configured incorrectly.
 
 ## Fix for Nginx
 
-Nginx has special variable `$realpath_root`, use it to set up `SCRIPT_FILENAME`:
+Nginx has special variable `$realpath_root`, use it to set up `SCRIPT_FILENAME` and `DOCUMENT_ROOT`:
 
 ```diff
 location ~ \.php$ {
   include fastcgi_params;
   fastcgi_pass unix:/var/run/php/php-fpm.sock;
 - fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
 + fastcgi_param SCRIPT_FILENAME $realpath_root$fastcgi_script_name;
+- fastcgi_param DOCUMENT_ROOT $document_root;
++ fastcgi_param DOCUMENT_ROOT $realpath_root;
 }
 ```