invalid invariants documented in `NetworkIndex.SetNode`

While working on https://github.com/hashicorp/nomad/pull/16401 a while back I noticed that [`NetworkIndex.SetNode`](https://github.com/hashicorp/nomad/blob/v1.5.6/nomad/structs/network.go#L211) documents some invariants that don't seem to be true but maybe aren't necessary either. This says they need to be idempotent but they're never actually called twice on the same instance as far as I can tell.

This needs some investigation and probable refactoring when we go thru and finally clean up the old network resource structs.

Start in nomad/structs/network.go at NetworkIndex.SetNode, then inspect its call sites to determine whether the documented idempotency invariant is exercised. Confirm whether the invariant is necessary and identify the old network resource structs involved. Done means the documentation and any required refactoring accurately reflect the observed usage.

Written by the indexing model from the issue text.