Add comment explaining negative block id test

tests/core/contracts/test_contract_util_functions.py

@@ -3,6 +3,11 @@
 )
 
 
+#  This tests negative block number identifiers, which behave like python
+#  list slices, with -1 being the latest block and -2 being the block before that.
+#  This test is necessary because transaction calls allow negative block indexes, although
+#  getBlock() does not allow negative block identifiers. Support for negative block identifier
+#  will likely be removed in v5.
 def test_parse_block_identifier_int(web3):
     last_num = web3.eth.getBlock('latest').number
     assert 0 == parse_block_identifier_int(web3, -1 - last_num)