ptr_ring: document usage around __ptr_ring_peek

This explains why is the net usage of __ptr_ring_peek
actually ok without locks.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>