Currently, authentication documentation states the following:

• In case of web server authentication all users (even with frontend access set to LDAP/Internal) will be authenticated by the web server, not by Zabbix!
• Be careful! Make sure that web server authentication is configured and works properly before switching it on.

We should minimize the risks caused by misconfiguration by introducing a hint for this param in Zabbix Frontend.

Additionally, a confirmation should be introduced to remind users that web-server configuration should be done in the first place.