add cross-references to pure evaluation mode

use consistent wording everywhere.
add some details on the configuration option documentation.

src/libutil/experimental-features.cc

@@ -50,6 +50,8 @@ constexpr std::array<ExperimentalFeatureDetails, 13> xpFeatureDetails = {{
             or other impure derivations can rely on impure derivations. Finally,
             an impure derivation cannot also be
             [content-addressed](#xp-feature-ca-derivations).
+
+            This is a more explicit alternative to using [`builtins.currentTime`](@docroot@/language/builtin-constants.md#builtins-currentTime).
         )",
     },
     {