Update towards-reproducibility-pinning-nixpkgs.md (#1029)

I expected the `<nixpkgs>` link to point to a resource that explains what that construct is and not directly to Nixpkgs. If I would be totally new to Nix, that would be even more confusing.

source/tutorials/first-steps/towards-reproducibility-pinning-nixpkgs.md

@@ -2,14 +2,19 @@
 
 # Towards reproducibility: pinning Nixpkgs
 
-In various Nix examples, you'll often see references to [\<nixpkgs>](https://github.com/NixOS/nixpkgs), as follows.
+In various Nix examples, you'll often see the following:
 
 ```nix
 { pkgs ? import <nixpkgs> {} }:
 
 ...
 ```
 
+:::{note}
+`<nixpkgs>` points to the file system path of some revision of {term}`Nixpkgs`.
+Find more information on [lookup paths](lookup-path-tutorial) in [](reading-nix-language).
+:::
+
 This is a **convenient** way to quickly demonstrate a Nix expression and get it working by importing Nix packages.
 
 However, <ref-search-path>**the resulting Nix expression is not fully reproducible**.