Document a known bug in the DES crypt cipher implementation which we're
not going to fix in order to stay compatible with legacy password data.
Nobody should use DES crypt anyway these days.
See http://www.freebsd.org/security/advisories/FreeBSD-SA-12:02.crypt.asc
for details about this bug.
Discussed with deraadt and beck about half a year ago (I'm pruning Ms
from my tree).