Document PciBusID format

While building [my Nomad Device Plugin](https://github.com/hashicorp/nomad/blob/8a4bd61caf747f50416c44d5a8b39eb06226ff9e/plugins/device/device.go#L149), I struggled to identify the exact format for [`DeviceLocality.PciBusID string`](https://github.com/hernad/nomad/blob/v1.6.112/plugins/device/device.go#L150).

I note that the [Nomad docs show GUIDs](https://github.com/hashicorp/nomad/blob/8a4bd61caf747f50416c44d5a8b39eb06226ff9e/website/content/api-docs/nodes.mdx#L379) like `"PciBusID": "77cda534-0660-2688-6c2e-ad6c62fc5ff3"`. But I think that's because that's what the unit tests used?

I couldn't find how the NVidia plugin did it since I don't have any NVidia devices and the `nvml` API docs didn't say.

I want to make sure I present a value that is used appropriately by the Nomad Scheduler...

I ended up [using values like](https://github.com/neomantra/nomad-onload/blob/main/internal/onload_device/probes.go#L83) this `0000:b1:00.0`, as that's what comes back from `lshw -businfo -class network`:

```

// "lshw -businfo -class network" sample output:
// Bus info Device Class Description
// =======================================================
// pci@0000:04:00.0 eth2 network NetXtreme BCM5720 Gigabit Ethernet PCIe
// pci@0000:04:00.1 eth3 network NetXtreme BCM5720 Gigabit Ethernet PCIe
// pci@0000:31:00.0 network BCM57412 NetXtreme-E 10Gb RDMA Ethernet Controller
// pci@0000:31:00.1 network BCM57412 NetXtreme-E 10Gb RDMA Ethernet Controller
// pci@0000:98:00.0 eth4 network BCM57412 NetXtreme-E 10Gb RDMA Ethernet Controller
// pci@0000:98:00.1 eth5 network BCM57412 NetXtreme-E 10Gb RDMA Ethernet Controller
// pci@0000:b1:00.0 eth0 network SFC9220 10/40G Ethernet Controller
// pci@0000:b1:00.1 eth1 network SFC9220 10/40G Ethernet Controller
```

If there is an expected format, please document it.

Overall, it was pretty easy to make that plugin, thanks!

Start with plugins/device/device.go, especially DeviceLocality.PciBusID, and compare its use with the examples in website/content/api-docs/nodes.mdx and the linked device-plugin implementation. Determine the accepted PCI bus identifier format and update the Nomad node API documentation with a representative value and explanation; done means plugin authors can identify the correct format without consulting external sources.

Written by the indexing model from the issue text.