chore(tvix/eval): Update module comment for value::string

This sentence is a little stale; let's just link to NixString directly
for the authoritative source of truth.

Change-Id: I64e065c4148d29702b09820a0e7724a65fae7c67
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12181
Autosubmit: aspen <root@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/value/string.rs

@@ -1,8 +1,7 @@
 //! This module implements Nix language strings.
 //!
-//! Nix language strings never need to be modified on the language
-//! level, allowing us to shave off some memory overhead and only
-//! paying the cost when creating new strings.
+//! See [`NixString`] for more information about the internals of string values
+
 use bstr::{BStr, BString, ByteSlice, Chars};
 use nohash_hasher::BuildNoHashHasher;
 use rnix::ast;