doc(category_theory): add a few docstrings (#2046)
* doc(category_theory): add a few docstrings
* Apply suggestions from code review
Co-Authored-By: Johan Commelin <johan@commelin.net>
Co-authored-by: Johan Commelin <johan@commelin.net>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
