[3.12] gh-110812: Isolating Extension Modules HOWTO: List GC-related gotchas (GH-111504) (GH-112148)

gh-110812: Isolating Extension Modules HOWTO: List GC-related gotchas (GH-111504)
(cherry picked from commit 985679f05d1b72965bfbed99d1499c22815375e4)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>