doc: IsFinalTx comment about nSequence & OP_CLTV

It's somewhat surprising that a transaction's nLockTime field is ignored
when all nSequence fields are final, so this change aims to clarify this
behavior and cross reference relevant details of OP_CHECKLOCKTIMEVERIFY.

Author: nothingmuch

src/consensus/tx_verify.cpp

@@ -20,6 +20,15 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime)
         return true;
     if ((int64_t)tx.nLockTime < ((int64_t)tx.nLockTime < LOCKTIME_THRESHOLD ? (int64_t)nBlockHeight : nBlockTime))
         return true;
+
+    // Even if tx.nLockTime isn't satisfied by nBlockHeight/nBlockTime, a
+    // transaction is still considered final if all inputs' nSequence ==
+    // SEQUENCE_FINAL (0xffffffff), in which case nLockTime is ignored.
+    //
+    // Because of this behavior OP_CHECKLOCKTIMEVERIFY/CheckLockTime() will
+    // also check that the spending input's nSequence != SEQUENCE_FINAL,
+    // ensuring that an unsatisfied nLockTime value will actually cause
+    // IsFinalTx() to return false here:
     for (const auto& txin : tx.vin) {
         if (!(txin.nSequence == CTxIn::SEQUENCE_FINAL))
             return false;

src/script/interpreter.cpp

@@ -1741,9 +1741,9 @@ bool GenericTransactionSignatureChecker<T>::CheckLockTime(const CScriptNum& nLoc
     if (nLockTime > (int64_t)txTo->nLockTime)
         return false;
 
-    // Finally the nLockTime feature can be disabled and thus
-    // CHECKLOCKTIMEVERIFY bypassed if every txin has been
-    // finalized by setting nSequence to maxint. The
+    // Finally the nLockTime feature can be disabled in IsFinalTx()
+    // and thus CHECKLOCKTIMEVERIFY bypassed if every txin has
+    // been finalized by setting nSequence to maxint. The
     // transaction would be allowed into the blockchain, making
     // the opcode ineffective.
     //