[3.12] Cleanup and clarify our hashlib docs. (GH-105624) (#105632)

Cleanup and clarify our hashlib docs. (GH-105624)

Clarify and improve our hashlib docs. Now with 50% less mess!
(cherry picked from commit 0d1d6ab966ff4e4d651d0dee0ec9a2f868ef42da)

Co-authored-by: Gregory P. Smith <greg@krypto.org>