[3.11] gh-116035: Document that both tzinfo and fold are ignored in comparisons if tzinfo is the same (GH-116187) (GH-116217)

This mostly restores information removed in c12240ed28aac6494750e00143bc550c4d6d8ad1 (GH-114749).
(cherry picked from commit 05b04903a14279421ecdc6522b8202822de6ebb5)

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