agda: add extended comment on semantics of Δ
agda: add extended comment on semantics of Δ

During my walk-throughs on the code, explaining this line was a bit
hard and required a small example. After writing it here during the
walkthrough, I decided to extend it and save it.

Old-commit-hash: 279b6047512b8c934cae49bda5977fa104077e12

inc-lc/ilc-agda