Update MSC1711 FAQ to be explicit about well-known (#4584)

A surprising number of people are using the well-known method, and are
simply copying the example configuration. This is problematic as the
example includes an explicit port, which causes inbound federation
requests to have the HTTP Host header include the port, upsetting some
reverse proxies.

Given that, we update the well-known example to be more explicit about
the various ways you can set it up, and the consequence of using an
explict port.