kadmin.8: Document the new dump -f flag

Commit 5000d023a446 added a new flag to the dump option.

This patch documents this new flag.

This is a content change.

Fixes: 5000d023a446 ("heimdal-kadmin: Add support for the -f dump option")

(cherry picked from commit 03da141d59ae1da4e66974c466e2cb26f296e6df)