roachpb: provide an explanation for Transaction.refreshed_timestamp

Release note: None

pkg/roachpb/data.proto

@@ -314,6 +314,11 @@ message Transaction {
   // encountered during the course of the txn. If set, this take precedence
   // over orig_timestamp and is the timestamp at which the transaction both
   // reads and writes going forward.
+  // We need to keep track of both refresh_timestamp and orig_timestamp (instead
+  // of simply overwriting the orig_timestamp after refreshes) because the
+  // orig_timestamp needs to be used as a lower bound timestamp for the
+  // time-bound iterator used to resolve intents - i.e. there can be intents to
+  // resolve up to the timestamp that the txn started with.
   util.hlc.Timestamp refreshed_timestamp = 15 [(gogoproto.nullable) = false];
   // A list of <NodeID, timestamp> pairs. The list maps NodeIDs to timestamps
   // as observed from their local clock during this transaction. The purpose of