revert anchor prefix for builtin constants

the original change broke many pre-existing anchor links.

also change formatting of the constants listing slightly:
- the type should not be part of the anchor
- add highlight to the "impure only" note

src/libexpr/eval.hh

@@ -806,7 +806,7 @@ struct EvalSettings : Config
           List of directories to be searched for `<...>` file references
 
           In particular, outside of [pure evaluation mode](#conf-pure-evaluation), this determines the value of
-          [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtin-constants-nixPath).
+          [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath).
         )"};
 
     Setting<bool> restrictEval{