The current example in the manual no longer builds, mainly because
`useDune2 = true` is required, but also because the inputs have changed.
The new examples are copied verbatim from nixpkgs.
* 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
