update pill 7.2 with `nix log`, add clarification

[ammkrn]

In pill 7.2, the terminal output of nix-repl> :b d no longer includes the build logs, which makes the exercise in 7.2 very difficult to understand. As tomberek on the discourse pointed out, the logs showing the output of declare -xp should be viewed with nix log <storepath>. I also added a little bit of explanation to hopefully make it more clear what's happening, and why we're using declare -xp to print variables.