docs: networking: netdevsim rules

There are aspects of netdevsim which are commonly
misunderstood and pointed out in review. Cong
suggest we document them.

Suggested-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>