[NET]: Document some simple rules for actions

This patch adds documentation on what is expected of an
action which branches away from the action-graph or when it
needs to trample on actins. It also describes what is expected of
users of such actions.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>