Setting the Root Device for Deployment in TripleO dead link

- [X] This doc is inaccurate in this way: _contains a dead/hanging/404 link_
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

-----------------------------------
Release: 3.0.0 on 2020-08-28 16:27:59
SHA: 9e7e14fd088e7857c4157e3c8d5f46ced8aae8aa
Source: https://opendev.org/openstack/tripleo-docs/src/deploy-guide/source/provisioning/root_device.rst
URL: https://docs.openstack.org/project-deploy-guide/tripleo-docs/latest/provisioning/root_device.html

The link to "Ironic root device hints documentation" goes to
https://docs.openstack.org/ironic/deploy/install-guide.html#specifying-the-disk-for-deployment
which yields 404.

That part of documentation is necessary in order to describe
additional attributes such as "name", because not every kind
of storage device has a WWN.