man: don't suggest using pam_unix.so's use_authtok switch

man: don't suggest using pam_unix.so's use_authtok switch

Our dumbed down example PAM stacks do not contain cracklib/pwq modules,
hence using use_authtok on the pam_unix.so password change stack won't
work, because it has the effect that pam_unix.so never asks for a
password on its own, expecting the cracklib/pwq modules to have
queried/validated them beforehand.

I noticed this issue because of #30969: Debian's PAM setup suffers by
the same issue – even though they don't actually use our suggested PAM
fragments at all.

See: #30969