bridge: add some comments for NETDEV_RELEASE

Add comments on why we don't notify NETDEV_RELEASE.

Cc: David Miller <davem@davemloft.net>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>