Add comment on most_recent_write

src/lock.rs

@@ -10,6 +10,11 @@ pub(crate) struct Lock<Id: TransactionId> {
     kind: AccessKind,
     // Current Reads or Write
     current_locks: SmallVec<[Id; 4]>,
+    // This can only be populated when the current lock is a read-lock, and
+    // there has been a previous write-lock.
+    // Tracks the most recent, but not current, write-lock.
+    // It is necessary to track, because all current read-locks will conflict
+    // with the most recent write-lock.
     most_recent_write: Option<Id>,
 }