Docs: Describe surprising side effects of auth_param basic (#1612)

Docs: Describe surprising side effects of auth_param basic (#1612)

    acl badGuys proxy_auth Bob
    http_access deny badGuys

Admins may be surprised that their proxy_auth ACLs do not match users
with logins identical to those listed as proxy_auth ACL values. For
example, a user logged in as "Bob" will no match the above ACL if Basic
authentication is used without an explicit "casesensitive on" setting.
In fact, the above ACL cannot match any user in that environment!