doc: reference NIX_ATTRS_*_FILE vars at the env var reference for drvs

doc/manual/src/language/derivations.md

@@ -165,6 +165,11 @@ The [`builder`](#attr-builder) is executed as follows:
   - `NIX_STORE` is set to the path of the top-level Nix store
     directory (typically, `/nix/store`).
 
+  - `NIX_ATTRS_JSON_FILE` & `NIX_ATTRS_SH_FILE` if `__structuredAttrs`
+    is set to `true` for the dervation. A detailed explanation of this
+    behavior can be found in the
+    [section about structured attrs](./advanced-attributes.md#adv-attr-structuredAttrs).
+
   - For each output declared in `outputs`, the corresponding
     environment variable is set to point to the intended path in the
     Nix store for that output. Each output path is a concatenation