refine comments

Author: dantengsky

src/query/service/src/sessions/query_ctx.rs

@@ -1601,9 +1601,11 @@ impl TableContext for QueryContext {
                 let existing_timestamp = txn_mgr.get_table_txn_begin_timestamp(table_id);
 
                 if let Some(existing_ts) = existing_timestamp {
-                    // If we already have a transaction timestamp for this table,
+                    // Defensively check that:
+                    // Inside an active transaction, if we already have a transaction timestamp for this table,
                     // ensure the new segment_block_timestamp is greater than or equal to it.
-                    // This maintains timestamp monotonicity within the transaction.
+                    // This maintains timestamp monotonicity within the transaction, which is crucial for
+                    // the safety of vacuum operation.
                     if table_meta_timestamps.segment_block_timestamp < existing_ts {
                         return Err(ErrorCode::Internal(format!(
                             "[QUERY-CTX] Transaction timestamp violation: table_id = {}, new segment timestamp {:?} is lesser than existing transaction timestamp {:?}",