Document the consensus rules for Section 3.6 (#3338)

zebra-chain/src/sapling/spend.rs

@@ -171,6 +171,20 @@ impl ZcashSerialize for Spend<PerSpendAnchor> {
 }
 
 impl ZcashDeserialize for Spend<PerSpendAnchor> {
+    /// # Consensus
+    ///
+    /// > The anchor of each Spend description MUST refer to some earlier
+    /// > block’s final Sapling treestate. The anchor is encoded separately in
+    /// > each Spend description for v4 transactions, or encoded once and shared
+    /// > between all Spend descriptions in a v5 transaction.
+    ///
+    /// <https://zips.z.cash/protocol/protocol.pdf#spendsandoutputs>
+    ///
+    /// This rule is also implemented in
+    /// [`zebra_state::service::check::anchor`] and
+    /// [`zebra_chain::transaction::serialize`].
+    ///
+    /// The "anchor encoding for v4 transactions" is implemented here.
     fn zcash_deserialize<R: io::Read>(mut reader: R) -> Result<Self, SerializationError> {
         Ok(Spend {
             cv: commitment::NotSmallOrderValueCommitment::zcash_deserialize(&mut reader)?,

zebra-chain/src/transaction/serialize.rs

@@ -224,6 +224,21 @@ impl ZcashDeserialize for Option<sapling::ShieldedData<SharedAnchor>> {
             .collect();
 
         // Create transfers
+        //
+        // # Consensus
+        //
+        // > The anchor of each Spend description MUST refer to some earlier
+        // > block’s final Sapling treestate. The anchor is encoded separately
+        // > in each Spend description for v4 transactions, or encoded once and
+        // > shared between all Spend descriptions in a v5 transaction.
+        //
+        // <https://zips.z.cash/protocol/protocol.pdf#spendsandoutputs>
+        //
+        // This rule is also implemented in
+        // [`zebra_state::service::check::anchor`] and
+        // [`zebra_chain::sapling::spend`].
+        //
+        // The "anchor encoding for v5 transactions" is implemented here.
         let transfers = match shared_anchor {
             Some(shared_anchor) => sapling::TransferData::SpendsAndMaybeOutputs {
                 shared_anchor,

zebra-consensus/src/transaction.rs

@@ -798,6 +798,13 @@ where
 
             let bvk = orchard_shielded_data.binding_verification_key();
 
+            // # Consensus
+            //
+            // > The Spend transfers and Action transfers of a transaction MUST be
+            // > consistent with its vbalanceSapling value as specified in § 4.13
+            // > ‘Balance and Binding Signature (Sapling)’ on p. 49.
+            //
+            // <https://zips.z.cash/protocol/protocol.pdf#spendsandoutputs>
             async_checks.push(
                 primitives::redpallas::VERIFIER
                     .clone()