Add documentation recommending contract keepers to defensively perform SendPacket validation in OnAcknowledgement and OnTimeout as well

[colin-axner]

Summary

Add documentation recommending contract keepers to defensively perform SendPacket validation in OnAcknowledgement and OnTimeout as well. This is to defend against chain's accidentally miss wiring their app.go (this is feasible given the subtle complexity to wiring an IBC stack)

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