[3.12] gh-105535 Document potential performance trap during enum creation (GH-107119) (GH116511)

(cherry picked from commit 601f3a7b3391e9d219a8ec44a6c56d00ce584d2a)

Co-authored-by: Nicolas A. Oyarzabal <79150521+nicky-eng@users.noreply.github.com>
Co-authored-by: Ethan Furman <ethan@stoneleaf.us>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>