feat: add --userdata-file to device/update

[dlotterman]

Some preferential doc sting changes to highlight the ease of the recent changes to updating always-pxe

Also C+V, C+V's the --userdata-file code from device/create to device/update, making it easier to update complicated userdata formats for devices in place.

$ bin/metal device update -i SOMEDEVICEUUID --userdata-file /tmp/my-cloud-init.mime
+--------------------------------------+----------+-------------+--------+
|                  ID                  | HOSTNAME |     OS      | STATE  |
+--------------------------------------+----------+-------------+--------+
| 9ffac3bb-4e04-466b-a786-b703fe52fa7f | h-01     | Custom iPXE | active |
+--------------------------------------+----------+-------------+--------+

Passed make generate-docs 🤷 ?

[displague]

@dlotterman I'm wondering if this PR was motivated by a bug that was recently addressed: #367 (comment)

A bare --always-pxe should have the behavior of enabling always-pxe, just as a bare --locked should lock the node.

When we were revisiting the Always PXE behavior, we did not look into if the Lock behavior had also been affected. (@cc @Vasubabu)