docs: Update go.mod rapid replace for clarity

The rapid package only used by Cosmos SDK tests so it shouldn't affect
kava. However, just including the replace and documenting it so it's
clear that we have verified the use of the package and not omitted
because we missed it.

go.mod

@@ -242,6 +242,6 @@ replace (
 	github.com/syndtr/goleveldb => github.com/informalsystems/goleveldb v0.0.0-20240822155617-8112573615f8
 	// Avoid change in slices.SortFunc, see https://github.com/cosmos/cosmos-sdk/issues/18415
 	golang.org/x/exp => golang.org/x/exp v0.0.0-20230711153332-06a737ee72cb
-	// stick with compatible version of rapid in v0.47.x line
+	// Rapid is only used for Cosmos SDK tests, including replace to match for clarity and explicitness
 	pgregory.net/rapid => pgregory.net/rapid v0.5.5
 )