Add missing docs on IBCReceiveResponse.Acknowledgement changes

docs/MIGRATING.md

@@ -14,6 +14,9 @@
 - The field `BlockInfo.Time` now uses a wrapper type `Uint64` instead of
   `uint64` to ensure string serialization. You can use `uint64(u)` to get the
   underlying value.
+- The field `IBCReceiveResponse.Acknowledgement` can now be `nil`. In this case,
+  no acknowledgement must be written. Callers need to handle this case
+  separately from empty data.
 - CosmWasm gas values were reduced by a factor of 1000, so each instruction now
   consumes 150 CosmWasm gas instead of 150000. This should be taken into account
   when converting between CosmWasm gas and Cosmos SDK gas.

types/ibc.go

@@ -256,7 +256,9 @@ type IBCReceiveResult struct {
 // and not inform the calling chain).
 // This is the counterpart of (IbcReceiveResponse)(https://github.com/CosmWasm/cosmwasm/blob/v0.15.0/packages/std/src/ibc.rs#L247-L267).
 type IBCReceiveResponse struct {
-	// binary encoded data to be returned to calling chain as the acknowledgement
+	// Acknowledgement is binary encoded data to be returned to calling chain as the acknowledgement.
+	// If this field is nil, no acknowledgement must be written. For contracts before CosmWasm 2.0, this
+	// was always a non-nil value. See also https://github.com/CosmWasm/cosmwasm/pull/1892.
 	Acknowledgement []byte `json:"acknowledgement"`
 	// Messages comes directly from the contract and is it's request for action.
 	// If the ReplyOn value matches the result, the runtime will invoke this