Add deleted docstrings to Cancun's transaction.py

src/ethereum/cancun/transactions.py

@@ -1,3 +1,8 @@
+"""
+Transactions are atomic units of work created externally to Ethereum and
+submitted to be executed. If Ethereum is viewed as a state machine,
+transactions are the events that move between states.
+"""
 from dataclasses import dataclass
 from typing import Tuple, Union