doc: recursive-nix: advertise requiredSystemFeatures

It's best we teach users that the "foo" derivation is less than pure in the sense that it cannot be built just on any system, in particular that builders cannot be selected arbitrarily but based on their system-features. The `"recursive-nix"` system-feature is automatically defined by `--extra-experimental-features recursive-nix`

src/libutil/experimental-features.cc

@@ -125,6 +125,8 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
 
             runCommand "foo"
               {
+                 # Optional: let Nix know "foo" requires the experimental feature
+                 requiredSystemFeatures = [ "recursive-nix" ];
                  buildInputs = [ nix jq ];
                  NIX_PATH = "nixpkgs=${<nixpkgs>}";
               }