docs: mention importNative/exec in allow-unsafe-native-code-during-evaluation (#10803)

* docs: mention importNative/exec in allow-unsafe-native-code-during-evaluation

Both of these still needs their own actual documentation, but they are
at least now mentioned that they exist and what they're enabled by.

Co-authored-by: Qyriad <qyriad@qyriad.me>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libexpr/eval-settings.hh

@@ -15,8 +15,24 @@ struct EvalSettings : Config
 
     static std::string resolvePseudoUrl(std::string_view url);
 
-    Setting<bool> enableNativeCode{this, false, "allow-unsafe-native-code-during-evaluation",
+    Setting<bool> enableNativeCode{this, false, "allow-unsafe-native-code-during-evaluation", R"(
-        "Whether builtin functions that allow executing native code should be enabled."};
+        Enable built-in functions that allow executing native code.
+
+        In particular, this adds:
+        - `builtins.importNative` *path*
+        
+          Load a dynamic shared object (DSO) at *path* which exposes a function pointer to a procedure that initialises a Nix language value, and return that value.
+          The procedure must have the following signature:
+          ```cpp
+          extern "C" typedef void (*ValueInitialiser) (EvalState & state, Value & v);
+          ``` 
+          
+          The [Nix C++ API documentation](@docroot@/contributing/documentation.md#api-documentation) has more details on evaluator internals.
+          
+        - `builtins.exec` *arguments*
+        
+          Execute a program, where *arguments* are specified as a list of strings, and parse its output as a Nix expression.
+    )"};
 
     Setting<Strings> nixPath{
         this, getDefaultNixPath(), "nix-path",