[3.13] gh-74955: Document that __all__ must contain strings in normalization form NFKC (GH-144504) (GH-144520)

(cherry picked from commit c81e1843d4bc0a51cf4f77d19b5ac4e49f714a0d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>