The old variables still work. The global lets us avoid
auto-detection logic like we're currently doing for
`matrix_nginx_proxy_proxy_matrix_federation_api_enabled`.

In the future, we'd just be able to reference
`matrix_homeserver_federation_enabled` and know the up-to-date value
regardless of homeserver.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2090

This is a proposed add to the documentation to inform users how to change the federation port using the ansible playbook for eventual use with third party services such as CDNs.

ma1sd requires the openid endpoints for certain functionality.
Example: 90b2b5301c/src/main/java/io/kamax/mxisd/auth/AccountManager.java (L67-L99)

If federation is disabled, we still need to expose these openid APIs on the
federation port.

Previously, we were doing similar magic for Dimension.
As per its documentation, when running unfederated, one is to enable
the openid listener as well. As per their recommendation, people
are advised to do enable it on the Client-Server API port
and use the `federationUrl` variable to override where the federation
port is (making federation requests go to the Client-Server API).

Because ma1sd always uses the federation port (unless you do some
DNS overwriting magic using its configuration -- which we'd rather not
do), it's better if we just default to putting the `openid` listener
where it belongs - on the federation port.

With this commit, we retain the "automatically enable openid APIs" thing
we've been doing for Dimension, but move it to the federation port instead.
We also now do the same thing when ma1sd is enabled.

Fixes #502 (Github Issue)

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)".