Polish: clarify authenticate_ip_ttl code

Polish: clarify authenticate_ip_ttl code

This patch alters the directive implementation to only perform TTL
addition when setting the expiry value. This improves speed a little when
comparing timestamps, and allows the config file to display 1 second TTL
instead of displaying 0 seconds and actually being 0-1 seconds.
 Which resolves some confusion about why max_user_ip ACL still works when
the TTL is set to 0 seconds.

Also, document the AuthUserIP class used to store the IP information.