Add some comments

Dependencies

• [2] FYBNOJLT Automagically add --rebuild to nix build commands in interactive mode
• [*] W2G433QU Refactor libdemo and flakeref for nix run commands
• [*] WNFTZY3M Optionally prevent relevant nix store paths from being GC'ed

Change contents

• edit in bash/nix-container-demo-helper.sh at line 57
  [5.916]

  [5.916]
  + # Wrapper for the nix executable to help with interactive demos and gc roots

  + # - If WRAP_NIX_MAKE_GCROOTS=1 the nix (build|develop|run) commands will add gc roots for their outputs

  + # - In interactive mode, nix build commands will be run with --rebuild for demonstration purposes, if its output already exists

  + # Assumptions: invocation is of the form 'nix <action> <flake output> ...'.
• edit in bash/nix-container-demo-helper.sh at line 68
  [5.1132]

  [2.436]
  + # Separate arguments for nix from arguments passed through to another command, separated by '--'
• edit in bash/nix-container-demo-helper.sh at line 81
  [5.1504]

  [2.764]
  + # Custom actions for nix run, build, and develop commands