Clarify network_instance in AFTOperation

[xw-g]

Explicitly call out that a AFTOperation needs to always have network_instance specified, because the following reasons:

• If we expect controller do the due diligent of maintaining right AFTOperation orderings, we should also expect the controller to know which instance it meant to modify.
• Keep the requirement/behavior consistent with other RPC calls. For example, both GetRequest and FlushRequest requires the network_instance to be explicitly specified.

[xw-g]

@robshakir I think we have briefly talked about this one.