[Software sources] clarify software sources when using channels

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way:

The doc states:

> When a charm is delivered via a channel, the software sources will be configured automatically.

But it doesn't clarify how this relates to the `openstack-origin` config option. Does the charm ignore the openstack-origin config value if delivered via a channel? Does it only configure software sources automatically if the openstack-origin config is empty?

Also note that the 'OpenStack upgrade' docs in the charm deployment guide ( https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/yoga/upgrade-openstack.html ) doesn't mention this auto-configuration of software sources.

If you have a troubleshooting or support issue, use the following resources:

 - The mailing list: https://lists.openstack.org
 - IRC: 'openstack' channel on OFTC

-----------------------------------
Release: 0.0.1.dev809 on 2022-10-17 17:37:26
SHA: f5d8641d96567a65cfba3852cdced1163cb82a85
Source: https://opendev.org/openstack/charm-guide/src/doc/source/concepts/software-sources.rst
URL: https://docs.openstack.org/charm-guide/latest/concepts/software-sources.html