[doc] Update comments on dummy extraNonces in tests

diff --git a/src/test/util/mining.cpp b/src/test/util/mining.cpp
index da15e678..05f1be77 100644
--- a/src/test/util/mining.cpp
+++ b/src/test/util/mining.cpp
@@ -49,6 +49,7 @@ std::vector<std::shared_ptr<CBlock>> CreateBlockChain(size_t total_height, const
         coinbase_tx.vout.resize(1);
         coinbase_tx.vout[0].scriptPubKey = P2WSH_OP_TRUE;
         coinbase_tx.vout[0].nValue = GetBlockSubsidy(height + 1, params.GetConsensus());
+        // Always include OP_0 as a dummy extraNonce.
         coinbase_tx.vin[0].scriptSig = CScript() << (height + 1) << OP_0;
         block.vtx = {MakeTransactionRef(std::move(coinbase_tx))};
 
diff --git a/src/test/validation_block_tests.cpp b/src/test/validation_block_tests.cpp
index 2303fd4c..dfa66bb8 100644
--- a/src/test/validation_block_tests.cpp
+++ b/src/test/validation_block_tests.cpp
@@ -82,7 +82,7 @@ std::shared_ptr<CBlock> MinerTestingSetup::Block(const uint256& prev_hash)
     txCoinbase.vout[1].nValue = txCoinbase.vout[0].nValue;
     txCoinbase.vout[0].nValue = 0;
     txCoinbase.vin[0].scriptWitness.SetNull();
-    // Always pad with OP_0 at the end to avoid bad-cb-length error
+    // Always pad with OP_0 as dummy extraNonce (also avoids bad-cb-length error for block <=16)
     const int prev_height{WITH_LOCK(::cs_main, return m_node.chainman->m_blockman.LookupBlockIndex(prev_hash)->nHeight)};
     txCoinbase.vin[0].scriptSig = CScript{} << prev_height + 1 << OP_0;
     txCoinbase.nLockTime = static_cast<uint32_t>(prev_height);
diff --git a/test/functional/test_framework/blocktools.py b/test/functional/test_framework/blocktools.py
index 8ad8ea8a..e1453e3a 100644
--- a/test/functional/test_framework/blocktools.py
+++ b/test/functional/test_framework/blocktools.py
@@ -164,7 +164,7 @@ def add_witness_commitment(block, nonce=0):
 def script_BIP34_coinbase_height(height):
     if height <= 16:
         res = CScriptOp.encode_op_n(height)
-        # Append dummy to increase scriptSig size to 2 (see bad-cb-length consensus rule)
+        # Append dummy extraNonce to increase scriptSig size to 2 (see bad-cb-length consensus rule)
         return CScript([res, OP_0])
     return CScript([CScriptNum(height)])