Storage: Clarify that device names such `/dev/sda` may not be stable and recommend using labels for partitions

As a first step toward improving things for https://github.com/coreos/fedora-coreos-tracker/issues/1732, we should audit our docs (and Butane's & Ignition's ones) to make sure that we explicitly note that disk devices names may not be stable across boots.

Partitioning using Ignition and mount points should thus use labels as much as possible to reference to disk partitions to make sure they are stable across boots.

Ideally we should provide examples to update existing setups / mount units.

See: https://github.com/openshift/os/blob/master/docs/faq.md#q-how-do-i-configure-a-secondary-block-device-via-ignitionmc-if-the-name-varies-on-each-node

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As a second step, we need to mention using truly stable names for disks in Ignition config and the corresponding downsides that we have for now.

See: https://github.com/coreos/fedora-coreos-tracker/issues/1732

Start by auditing the Fedora CoreOS documentation for storage, partitioning, and mount points, then compare the corresponding Butane and Ignition documentation. Use the linked OpenShift FAQ and Fedora CoreOS tracker issue as references; the work is done when the docs explain device-name instability, recommend stable labels or names, and include guidance for existing setups where appropriate.