docs: networking: timestamping: update for DSA switches

Update timestamping doc for DSA switches to describe current
implementation accurately. On TX, the skb cloning is no longer
in DSA generic code.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>