docs: document set theory symbols
docs: document set theory symbols

These symbols confused me so I wanted to gloss them to help future users. You can see the context here: https://discourse.nixos.org/t/flakes-what-does-mean-big-bundle-of-questions

NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix