Apply suggestions from code review

Co-authored-by: str4d <jack@electriccoin.co>

zcash_primitives/src/transaction/components/sapling/builder.rs

@@ -225,6 +225,7 @@ impl<P: consensus::Parameters> SaplingBuilder<P> {
         }
     }
 
+    /// Returns the net value represented by the spends and outputs added to this builder.
     pub fn value_balance(&self) -> Amount {
         self.value_balance
     }

zcash_primitives/src/transaction/mod.rs

@@ -181,7 +181,7 @@ impl TxVersion {
             BranchId::Sapling | BranchId::Blossom | BranchId::Heartwood | BranchId::Canopy => {
                 TxVersion::Sapling
             }
-            BranchId::Nu5 => TxVersion::ZFuture,
+            BranchId::Nu5 => unimplemented!(),
             #[cfg(feature = "zfuture")]
             BranchId::ZFuture => TxVersion::ZFuture,
         }