"Using DNS with Neutron & Nova in designate" documentation is insufficient

[Marcus Boden]

The documentation states that:
> You can check if the dns-domain-ports extension is enabled by querying the Neutron extensions API:
>
> openstack extension list --network -f value -c Alias | grep dns-domain-ports
> dns-domain-ports

This is incorrect. You only see if it is available, not if it is actually activated:
$ openstack extension list --network -f value -c Alias | grep subnet-dns-publish-fixed-ip
subnet-dns-publish-fixed-ip
$ juju config neutron-api enable-ml2-dns
true
[on neutron-api unit]
# grep extension /etc/neutron/plugins/ml2/ml2_conf.ini
extension_drivers=port_security,dns_domain_ports

The cloud in question is on Yoga.

----
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- [x] This doc is inaccurate in this way: see above
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: 18.1.0.dev6 on 2022-01-19 02:42:29
SHA: f318d735ed02fe39bbfd5fd4b85b647f0843a7a6
Source: https://opendev.org/openstack/designate/src/doc/source/user/neutron-integration.rst
URL: https://docs.openstack.org/designate/latest/user/neutron-integration.html