Relay hostnames that have MX records are looked up by postfix
and the MX record's payload is used instead.

This special behavior may be undesirable, so we make sure to
point it out.

This makes it explicit that outgoing traffic (25/587) needs
to be let through, as well as documenting how to debug
other non-delivery issues.

This is provoked by Github issue #46.

No client had made use of the well-known mechanism
so far, so the set up performed by this playbook was not tested
and turned out to be a little deficient.

Even though /.well-known/matrix/client is usually requested with a
simple request (no preflight), it's still considered cross-origin
and [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS)
applies. Thus, the file always needs to be served with the appropriate
`Access-Control-Allow-Origin` header.

Github issue #46 attempts to fix it at the "reverse-proxying" layer,
which may work, but would need to be done for every server.
It's better if it's done "upstream", so that all reverse-proxy
configurations can benefit.

We've had some people get confused into installing
Matrix Corporal and having pain with that.

With this documentation change, we try to make it clearer
that it's an advanced feature not to be touched unless
you know what you're doing.

On a similar note, we also make sure other things are properly
labeled as "(optional)" and/or "(advanced)".

It isn't below anymore

Missing file ending results in 404

It's been incorrect since 23e4a473