libexpr: Add release note for c-api-byidx change

Author: xokdvium

doc/manual/rl-next/c-api-byidx.md

@@ -0,0 +1,7 @@
+---
+synopsis: "C API: `nix_get_attr_name_byidx`, `nix_get_attr_byidx` take a `nix_value *` instead of `const nix_value *`"
+prs: [13987]
+---
+
+In order to accommodate a more optimized internal representation of attribute set merges these functions require
+a mutable `nix_value *` that might be modified on access. This does *not* break the ABI of these functions.