smbfs: caution against use in the manpage

It supports only the obsolete SMBv1 protocol, is known to be buggy, and
likely has security vulnerabilities. It will either be updated or
removed in the future, but for now at least describe the current state
in the man page.

PR: 263043
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 7c140ef034c33ab0b16b535f3d5f945dc8f71cb4)