PLT-4169 Added note on use of foldr in semantics.

This addresses the audit comment:

> Helper function convertReduceWarnings, lines 617 (Semantics.hs),
> 537 (Semantics.thy) The Haskell function is implemented using
> foldr, while the Isabelle function uses explicit recursion,
> making a oneto-one comparison less obvious. If there is a
> reason for this discrepancy, such as foldr yielding some
> optimizations, this should be outlined in a comment.

marlowe/src/Language/Marlowe/Core/V1/Semantics.hs

@@ -739,7 +739,7 @@ applyInput _ _ _ _                          = ApplyNoMatchError
 
 -- | Propagate 'ReduceWarning' to 'TransactionWarning'.
 convertReduceWarnings :: [ReduceWarning] -> [TransactionWarning]
-convertReduceWarnings = foldr (\warn acc -> case warn of
+convertReduceWarnings = foldr (\warn acc -> case warn of  -- Note that `foldr` is used here for efficiency, differing from Isabelle.
     ReduceNoWarning -> acc
     ReduceNonPositivePay accId payee tok amount ->
         TransactionNonPositivePay accId payee tok amount : acc