invalid invariants documented in NetworkIndex.SetNode

[tgross]

While working on #16401 a while back I noticed that NetworkIndex.SetNode 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.