Document pnpm prune

[Kinrany]

What does pnpm prune actually do?

I assume it's similar to npm prune, but pnpm is sufficiently different that specific behavior is not obvious. Plus understanding its function requires reading npm docs.

Right now pnpm docs just say that it "removes unnecessary packages". What exactly does "unnecessary" mean?

I started looking at it because I want to delete all cached files for a project without deleting the project itself -- both node_modules and all the packages in store not used by other projects, similar to Rust's cargo clean. I assume pnpm prune doesn't do that?