doc/using/configuration: document NIXPKGS_CONFIG lookup order (#515272)

doc/using/configuration.chapter.md

@@ -31,6 +31,22 @@ Unfree software is not tested or built in Nixpkgs continuous integration, and th
 Most unfree licenses prohibit either executing or distributing the software.
 :::
 
+The `NIXPKGS_CONFIG` environment variable can override the configuration file location.
+Nixpkgs resolves the config in this order:
+
+1.  `$NIXPKGS_CONFIG`, if set and the file exists.
+2.  `~/.config/nixpkgs/config.nix`, if it exists.
+3.  `~/.nixpkgs/config.nix` (legacy), if it exists.
+4.  Empty configuration.
+
+On NixOS, `NIXPKGS_CONFIG` points to `/etc/nix/nixpkgs-config.nix` system-wide.
+Drop a file there to apply configuration to `nix-env`, `nix-shell`, and other user-level commands.
+NixOS does not create this file.
+The [`nixpkgs.config`](https://nixos.org/manual/nixos/stable/options#opt-nixpkgs.config) option does not affect `nix-env`, `nix-shell`, or other user-level commands.
+
+This lookup applies to non-flake usage like channels and `<nixpkgs>`.
+Flakes ignore it; pass `config` directly when importing `nixpkgs`.
+
 ## Installing broken packages {#sec-allow-broken}
 
 There are several ways to try compiling a package which has been marked as broken.