There is more documentation coming in Expand manual on derivation outputs by Ericson2314 · Pull Request #12442 · NixOS/nix · GitHub
I wonder if it is a good idea to introduce there close to the head-line an info-box on how it differs from
It does link the protocols pages. It doesn’t refer to the Nix language because nothing in the store layer cares about the Nix language — this layering is introduced earlier in the manual when going over the architecture, not not restated for each store layer concept.
Rather, the documentation for buildins.derivation is now getting smaller, because relative information is moved to the store layer, and it links that instead. So in the other direction (because the language is definitely aware of the store layer, unlike the store layer not caring about the language) the difference and reliance should be very clear.