bpo-42982: Improve the text on suggested number of iterations of PBKDF2 (GH-24276)

Less specific number wording (as there is no one right number - the old 100k is too big for some applications and woefully small for others). We now link to NIST SP 800-132 to tell people what to read in there on how to decide for their application.

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