docs: fixup description of builtins.importNative

There was an argument missing and the fact that the imported function is
called.

src/libexpr/eval-settings.hh

@@ -19,18 +19,18 @@ struct EvalSettings : Config
         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.
+        - `builtins.importNative` *path* *symbol*
+          Runs function with *symbol* from a dynamic shared object (DSO) at *path*.
+          This may be used to add new primitive functions to the Nix language, that may not be possible to express within the language itself.
           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.
     )"};