Bug#1095686: openssh-server: Restore sshd_config(5) documentation about rdomain

On Mon, Feb 10, 2025 at 07:07:29PM +0100, Adam Lackorzynski wrote:
> in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=998069 the info about
> rdomain has been removed from the man page, indicating this is an OpenBSD-only
> feature.
> However, rdomain is also implemented for Linux: https://salsa.debian.org/ssh-team/openssh/-/blob/master/openbsd-compat/port-net.c?ref_type=heads#L49
> 
> Please allow the rdomain info to be back to the man-page.

We probably still at least need to remove the cross-reference to the
rdomain(4) manual page, since as far as I can tell that doesn't exist on
Debian.  Or is there some other suitable cross-reference that we could
substitute instead?  ip-vrf(8) looks potentially relevant.