Right yeah, I'll add a docstring.

I don't think they're necessarily mutually exclusive; it's certainly advanced usage, but it should be possible to make a model, add in some (backend specific) stuff, and pass that and a network to the builder, which builds the network as if the stuff that you added manually wasn't there.

The reason I did this was to make the `test_simulator` tests possible without builder hacks, but it seems like there's the possibility that this would be useful to backend designers.

I don't think that it makes any non-advanced usage more difficult, as the `model` kwarg is as opaque as the `builder` kwarg to novices.