bpo-42560: add warning to Tkinter docs about outdated pre-8.5 documentation online (GH-27836)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: E-Paine <63801254+E-Paine@users.noreply.github.com>
(cherry picked from commit 1eb451031a64b4f28d737ee7f394252ae9a76118)

Co-authored-by: Mark Roseman <mark@markroseman.com>