[3.12] gh-105766: Document that Custom Allocators Must Be Thread-Safe (gh-107519) (gh-107522)

gh-105766: Document that Custom Allocators Must Be Thread-Safe (gh-107519)
(cherry picked from commit db361a340af3970c279908c8746a6b9ed45f47b8)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>