[3.13] gh-134837: Correct and improve base85 documentation for base64 module (GH-145843) (GH-149743) (GH-149893)

(cherry picked from commit e667d62f114b54dcba17bdfad3835b9c91fda348)
(cherry picked from commit 9ad8a1b955033ebbf05456de2cfad85d1de2503d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: David Huggins-Daines <dhd@ecolingui.ca>