docs: Document the real behaviour of suspend-to-{mem,disk}
    
    We get a question every now and then about why hibernation works when
    suspend-to-disk is disabled and similar.  Let's hope that, by documenting the
    obvious more blatantly, people will get more informed.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>