man: Add note about single node configuration

Internally knet is using just one link for localhost so for single node
configuration knet_link_get_link_list returns only one entry. This is
propagated to corosync-cfgtool -s.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>