Specifically,
- include system-wide Nixpkgs channel so `NIX_PATH` solution also
works on multi-user installations;
- additionally link to Nix Discourse reply for more information on
`NIX_PATH`; and
- remove note about relog on NixOS since all the channels in
`~/.nix-defexpr/channels/` are included by default, so the Home
Manager channel should be picked up automatically.
The docs implied that fish was not really supported, but if fish is
managed by Home Manager, the generated config does use fenv to source
the session-vars file. Update the installation instructions and FAQ to
mention that fish does work, and mention fenv in the README.
