Merge pull request onflow#5378 from onflow/alex/cdp_doc

minor doc update for Chunk Data Pack

model/flow/chunk.go

@@ -60,14 +60,25 @@ func (ch *Chunk) Checksum() Identifier {
 
 // ChunkDataPack holds all register touches (any read, or write).
 //
-// Note that we have to capture a read proof for each write before updating the registers.
-// `Proof` includes proofs for all registers read to execute the chunk.
+// Note that we have to include merkle paths as storage proof for all registers touched (read or written) for
+// the _starting_ state of the chunk (i.e. before the chunk computation updates the registers).
+// For instance, if an execution state contains three registers: { A: 1, B: 2, C: 3}, and a certain
+// chunk has a tx that assigns A = A + B, then its chunk data pack should include the merkle
+// paths for { A: 1, B: 2 } as storage proof.
+// C is not included because it's neither read or written by the chunk.
+// B is included because it's read by the chunk.
+// A is included because it's updated by the chunk, and its value 1 is included because it's
+// the value before the chunk computation.
+// This is necessary for Verification Nodes to (i) check that the read register values are
+// consistent with the starting state's root hash and (ii) verify the correctness of the resulting
+// state after the chunk computation. `Proof` includes merkle proofs for all touched registers
+// during the execution of the chunk.
 // Register proofs order must not be correlated to the order of register reads during
 // the chunk execution in order to enforce the SPoCK secret high entropy.
 type ChunkDataPack struct {
 	ChunkID    Identifier      // ID of the chunk this data pack is for
 	StartState StateCommitment // commitment for starting state
-	Proof      StorageProof    // proof for all registers read during the chunk execution
+	Proof      StorageProof    // proof for all registers touched (read or written) during the chunk execution
 	Collection *Collection     // collection executed in this chunk
 
 	// ExecutionDataRoot is the root data structure of an execution_data.BlockExecutionData.