public-subnet not explained

In https://docs.openstack.org/neutron/rocky/admin/config-address-scopes.html, the public-subnet and ipv6-public-subnet pop up from nowhere, which is confusing. If these subnets are automatically generated, it should be explained. If these need to be manually created, the step should be added to the document.

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:

- [ ] This doc is inaccurate in this way: ______
- [x] 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: 13.0.2.dev5 on 2018-09-18 02:35
SHA: 517918dc5e73a115851d8a8d940f10bee295b315
Source: https://git.openstack.org/cgit/openstack/neutron/tree/doc/source/admin/config-address-scopes.rst
URL: https://docs.openstack.org/neutron/rocky/admin/config-address-scopes.html