Clarify that `http` can only be used for 127.0.0.1 for testing

https://tpo.pages.torproject.net/network-health/sbws/man_sbws.ini.html says that both http and https can be used, as pointed out by https://forum.torproject.org/t/destination-for-sbws-using-http/8692, but it doesn't clarify that it'll only accept 127.0.0.1 with http.