[3.11] Add an example of of custom `__repr__` (GH-112761) (#115901)

Added to repr entry in Doc/library/functions.rst.

---------

(cherry picked from commit 5770006ffac2abd4f1c9fd33bf5015c9ef023576)

Co-authored-by: Oh seungmin <tmdals179@gmail.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>