Update documentation to be more helpful about allow_weak_crypto.

https://github.com/krb5/krb5/commit/1619c3786c5b874b5fd88adc1f2fbaa50f2a95e4
Commit By: tlyu
Revision: 23750
Changed Files:
U   trunk/doc/admin.texinfo
U   trunk/src/config-files/krb5.conf.M