Debian/Ubuntu: Expand SSH instructions

This makes the PermitRootLogin changes part of the actual steps rather
than a comment.  I've also made its removal an explicit step.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/Debian/Debian Buster Root on ZFS.rst

@@ -804,9 +804,8 @@ Step 6: First Boot
 
      apt install --yes openssh-server
 
-   If you want to login as root via SSH, set ``PermitRootLogin yes`` in
-   ``/etc/ssh/sshd_config``.  For security, undo this as soon as possible
-   (i.e. once you have your regular user account setup).
+     vi /etc/ssh/sshd_config
+     # Set: PermitRootLogin yes
 
 #. Optional: Snapshot the initial installation::
 
@@ -960,6 +959,15 @@ Step 9: Final Cleanup
 
      sudo usermod -p '*' root
 
+#. Optional (but highly recommended): Disable root SSH logins:
+
+   If you installed SSH earlier, revert the temporary change::
+
+     vi /etc/ssh/sshd_config
+     # Remove: PermitRootLogin yes
+
+     systemctl restart ssh
+
 #. Optional: Re-enable the graphical boot process:
 
    If you prefer the graphical boot process, you can re-enable it now. If