Document what names are not possible as machine names.

[tomprince]

Given that network is used for specifying the description of deployment, as well as the pkgs to use, that somewhat obviously can't be used.

However, it additionally (appears)[https://github.com/DBCDK/morph/blob/ad13802e59c90d3a6d6c33a0a7cc1ffafbe151af/data/eval-machines.nix#L36) like defaults, resources and require (none of which appear to be used for anything[1]) as well as _file (which is passed to the module system) can't be used as machine names.

Alternatively, the structure could be inverted, so the contents of network is at the top-level, with a nodes key containing all the nodes. Though this would be a backwards incompatible change.

[1] I guess maybe these special attributes were inherited from nixops?