net: doc : use more suitable word 'unexpected' to replace 'secluded'

 'secluded' is used to describe places, not suitable here.

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