Clarify nLockTime-by-time comment in CheckFinalTx()

Author: petertodd

src/main.cpp

@@ -670,10 +670,11 @@ bool CheckFinalTx(const CTransaction &tx, int flags)
     // IsFinalTx() with one more than chainActive.Height().
     const int nBlockHeight = chainActive.Height() + 1;
 
-    // Timestamps on the other hand don't get any special treatment,
-    // because we can't know what timestamp the next block will have,
-    // and there aren't timestamp applications where it matters.
-    // However this changes once median past time-locks are enforced:
+    // BIP113 will require that time-locked transactions have nLockTime set to
+    // less than the median time of the previous block they're contained in.
+    // When the next block is created its previous block will be the current
+    // chain tip, so we use that to calculate the median time passed to
+    // IsFinalTx() if LOCKTIME_MEDIAN_TIME_PAST is set.
     const int64_t nBlockTime = (flags & LOCKTIME_MEDIAN_TIME_PAST)
                              ? chainActive.Tip()->GetMedianTimePast()
                              : GetAdjustedTime();