gh-99991: improve docs on str.encode and bytes.decode (GH-100198)

(cherry picked from commit a2bb3b7f9d8d15c81b724726454d68357fb31d1c)

Co-authored-by: Bisola Olasehinde <horlasehinde@gmail.com>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>