Update obsolete comment that still referred to CheckpointLock

CheckpointLock was removed in commit d18e75664a, and commit ce197e91d0
updated a leftover comment in CreateCheckPoint, but there was another
copy of it in CreateRestartPoint still.

src/backend/access/transam/xlog.c

@@ -9664,7 +9664,7 @@ CreateRestartPoint(int flags)
 	if (EnableHotStandby)
 		TruncateSUBTRANS(GetOldestTransactionIdConsideredRunning());
 
-	/* Real work is done, but log and update before releasing lock. */
+	/* Real work is done; log and update stats. */
 	LogCheckpointEnd(true);
 
 	/* Reset the process title */