doc: add function argument order convention (#110060)

* doc: add function argument order convention

Ordering by usage is the de facto ordering given to arguments. It's
logical, and makes finding argument usage easier. Putting lib first is
common in NixOS modules, so it's reasonable to mirror this in nixpkgs
proper. Additionally, it's not a package as such, has zero dependencies,
and can be found used anywhere in a derivation.

* doc: clean up usage of lib

doc/languages-frameworks/r.section.md

@@ -32,14 +32,12 @@ However, if you'd like to add a file to your project source to make the
 environment available for other contributors, you can create a `default.nix`
 file like so:
 ```nix
-let
-  pkgs = import <nixpkgs> {};
-  stdenv = pkgs.stdenv;
-in with pkgs; {
+with import <nixpkgs> {};
+{
   myProject = stdenv.mkDerivation {
     name = "myProject";
     version = "1";
-    src = if pkgs.lib.inNixShell then null else nix;
+    src = if lib.inNixShell then null else nix;
 
     buildInputs = with rPackages; [
       R