Improve D8+ nginx doc especially with cookie

In our company we already use a cookie check to switch auth request on our daily used test systems. I will work on solution / example with a kind of "@"-section switch I think.

This mapping helps to determine if a drupal 8+ cookie is available:

map $http_cookie $drupal_cookie {
  default 0;
  ~SSESS 1; # PHP session
}

See this post for example (with "~SESS" for older drupal versions: https://www.webfoobar.com/node/28