[3.14] gh-134201: Expand explanation of Base85 encodings in base64 docs (GH-134288) (#134298)

gh-134201: Expand explanation of Base85 encodings in base64 docs (GH-134288)

Explain history of de-facto standard and how to pick between the two Base-85 encoding functions in the base-64 module.

---------
(cherry picked from commit 66aaad61037785639aec393be7618cb54b1372dc)

Co-authored-by: Alek Binion <aleksander.binion@gmail.com>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>