[3.12] Add an example of of custom `__repr__` (GH-112761) (#115900)

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

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(cherry picked from commit 5770006ffac2abd4f1c9fd33bf5015c9ef023576)

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