Warn about dump -recurse nonfunctionality

kdb5_util dump -recurse hasn't behaved as documented since krb5-1.5,
when the DAL was integrated.  Restoring it is a nontrivial amount of
work, so just document it for now.

https://github.com/krb5/krb5/commit/eb8dc865efec4938d74a7955fdcd02bbee4c22b9
Author: Tom Yu <tlyu@mit.edu>
Commit: eb8dc865efec4938d74a7955fdcd02bbee4c22b9
Branch: master
 doc/admin/admin_commands/kdb5_util.rst |    6 ++++++
 src/kadmin/dbutil/dump.c               |    3 +++
 2 files changed, 9 insertions(+), 0 deletions(-)