Dropped node_id from docs, per #675

[ketzacoatl]

Specifically, see https://github.com/hashicorp/nomad/blob/master/CHANGELOG.md#030

Node ID is no longer specifiable. For users who have set a custom Node ID, the node should be drained before Nomad is updated and the data_dir should be deleted before starting for the first time [GH-675]

Thanks to @iverberk for the guidance in #1056

[dadgar]

Thanks!

[github-actions]

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