Add more context to wildcard type (`*`) explanation

[ElrohirGT]

Fixes https://github.com/roc-lang/roc/issues/5038

This PR adds a little bit more context to the wildcard type as discussed in the zullip chat.

It also adds the roc-cli package to the devShell in the flake.nix. This was done because the www/build-dev-local.sh bash script needs roc to run.