chore(portfolio-contract): Improve "toUSDN" comments

Ref Agoric/agoric-private#433

Author: gibson042

packages/portfolio-contract/tools/plan-solve.ts

@@ -446,9 +446,12 @@ export const rebalanceMinCostFlowSteps = async (
           break;
         }
         case 'toUSDN': {
-          // NOTE USDN transfer incurs a fee on output amount in basis points
-          // HACK of subtract 1n in order to avoid rounding errors in Noble
-          // See https://github.com/Agoric/agoric-private/issues/415
+          // USDN transfer imposes a "haircut" fee on the *amount*.
+          // Expose the post-fee net output as `detail.usdnOut`.
+          // TODO: Apply this in both directions per
+          // https://github.com/Agoric/agoric-private/issues/433
+          // HACK: subtract 1n to avoid rounding errors in Noble per
+          // https://github.com/Agoric/agoric-private/issues/415
           const usdnOut =
             (BigInt(flow) * (10000n - BigInt(edge.variableFee))) / 10000n - 1n;
           details = { detail: { usdnOut } };