update documentation for Nginx auth_request mode
update documentation for Nginx auth_request mode

18F/oauth2_proxy,bitly/google_auth_proxy,ploxiln/oauth2_proxy,bitly/google_auth_proxy,ploxiln/oauth2_proxy,jehiah/oauth2_proxy,Shopify/google_auth_proxy,Shopify/google_auth_proxy,bitly/oauth2_proxy,18F/oauth2_proxy,andyuc88/oauth2_proxy_azure,bitly/oauth2_proxy,jehiah/oauth2_proxy,Shopify/google_auth_proxy,andyuc88/oauth2_proxy_azure,snormore/oauth2_proxy,kalibrr/oauth2_proxy,snormore/oauth2_proxy,kalibrr/oauth2_proxy