* 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
With the addition of ruby.withPackages, manveru rewrote the nixpkgs
manual section for the ruby language but did not add it to the manual.
This commit replaces the previous documentation with manveru's updated
version.
