[3.13] gh-120452: improve documentation about private name mangling (GH-120451) (#121715)

gh-120452: improve documentation about private name mangling (GH-120451)
(cherry picked from commit f4d6e45c1e7161878b36ef9e876ca3e44b80a97d)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>