This documents the fact that nix-build, nix-env, nix-instantiate, and
nix-shell accept an extended syntax for their file arguments, including
some well-known (but not well documented) aspects, like being able to
specify `<nixpkgs>`, but also https:// tarball URLs, `flake:` prefixed
flakerefs, and the cursed `channel:` prefixed hardcoded URLs
Same thing for the new CLI incoming :)
Change-Id: Ib6d68594a16132805ba5d97526e16f7b3633117e
