Apply suggestions from code review

Co-authored-by: Daira Emma Hopwood <[email protected]>

zcash_client_backend/proto/proposal.proto

@@ -19,7 +19,7 @@ message Proposal {
     // The fee rule used in constructing this proposal
     FeeRule feeRule = 6;
     // The minimum number of confirmations required for transaction inputs to be selected.
-    // Each of the proposed inputs is know to have at least this many confirmations.
+    // Each of the proposed inputs must have at least this many confirmations.
     uint32 minConfirmations = 7;
     // The target height for which the proposal was constructed
     //
@@ -71,8 +71,7 @@ enum FeeRule {
     Zip317 = 3;
 }
 
-// The total value, fee amount, and change outputs of the proposed
-// transaction
+// The proposed change outputs and fee amount.
 message TransactionBalance {
     repeated ChangeValue proposedChange = 1;
     uint64 feeRequired = 2;

zcash_client_backend/src/fees.rs

@@ -54,6 +54,9 @@ impl ChangeValue {
 pub struct TransactionBalance {
     proposed_change: Vec<ChangeValue>,
     fee_required: NonNegativeAmount,
+
+    // A cache for the sum of proposed change and fee; we compute it on construction anyway, so we
+    // cache the resulting value.
     total: NonNegativeAmount,
 }