doc, net: Update documentation of synchronisation for TX multiqueue

Commits e308a5d8 ('netdev: Add
netdev->addr_list_lock protection.') and
e8a0464c

 ('netdev: Allocate multiple
queues for TX.') introduced more fine-grained locks.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>