Documentation : Make it clearer how to move SSH details from primary to secondary

In https://docs.gitlab.com/ee/administration/geo/replication/configuration.html#step-2-manually-replicate-primary-ssh-host-keys:

4. For each file in that list replace the file from the primary node to the same location on your secondary node.

This could be expanded a bit.

During the demo, we did this:

monozok@Michaels-MBP:~$ ssh -A mkozono@the_primary_ip
mkozono@primary:~$ sudo -E rsync /etc/ssh/ssh_host_* mkozono@the_secondary_ip:
mkozono@primary:~$ ssh mkozono@the_secondary_ip
mkozono@secondary:~# sudo mv /home/mkozono/ssh_host_* /etc/ssh/