net: Add comment about pernet_operations methods and synchronization

Make locking scheme be visible for users, and provide
a comment what for we are need exit_batch() methods,
and when it should be used.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 6056415d3a513846f774e7bbee0de0460b1c15df)
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>