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

[travier]

As a first step toward improving things for coreos/fedora-coreos-tracker#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.

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See: coreos/fedora-coreos-tracker#1732