Update fork.py

Author: SamWilsn

src/ethereum/cancun/fork.py

@@ -487,7 +487,12 @@ def process_system_transaction(
     data: Bytes,
 ) -> MessageCallOutput:
     """
-    Process a system transaction.
+    Process a system transaction with the given code.
+
+    Prefer calling `process_checked_system_transaction` or
+    `process_unchecked_system_transaction` depending on
+    whether missing code or an execution error should cause
+    the block to be rejected.
 
     Parameters
     ----------