[3.13] gh-121160: Note that readline libraries using different history formats. (GH-121327) (GH-122030)

This is not something we can do too much about, without help from the
underlying libraries.
(cherry picked from commit 709db44255eb5d73fc22a1341dd0253e71ddfda9)

Co-authored-by: Petr Viktorin <encukou@gmail.com>