Document default nix-path value

src/libexpr/eval.hh

@@ -581,7 +581,14 @@ struct EvalSettings : Config
 
     Setting<Strings> nixPath{
         this, {}, "nix-path",
-        "List of directories to be searched for `<...>` file references."};
+        R"(
+          List of directories to be searched for `<...>` file references.
+
+          If [pure evaluation](#conf-pure-eval) is disabled,
+          this is initialised using the [`NIX_PATH`](@docroot@/command-ref/env-common.md#env-NIX_PATH)
+          environment variable, or, if it is unset and [restricted evaluation](#conf-restrict-eval)
+          is disabled, a default search path including the user's and `root`'s channels.
+        )"};
 
     Setting<bool> restrictEval{
         this, false, "restrict-eval",