The Debian packaging's debconf template refers to the file ENCRYPTION.html which is meant to be in the samba-doc package, but it does not exist. It doesn't seem to exist in the source package, either. "See /usr/share/doc/samba-doc/htmldocs/ENCRYPTION.html from the samba-doc package for more details." - from packaging/Debian/debian-unstable/samba.templates
Samba has handed packaging back to the distributions, so this would be a Debian bug. But it looks like that has been removed in recent time (I couldn't work out when).