Add a note to Win32 gettimeofday() emulation, per Qingqing Zhou.

src/port/gettimeofday.c

@@ -2,7 +2,7 @@
  * gettimeofday.c
  *	  Win32 gettimeofday() replacement
  *
- * $PostgreSQL: pgsql/src/port/gettimeofday.c,v 1.6 2005/05/16 05:52:13 neilc Exp $
+ * $PostgreSQL: pgsql/src/port/gettimeofday.c,v 1.7 2005/12/16 21:55:27 alvherre Exp $
  *
  * Copyright (c) 2003 SRA, Inc.
  * Copyright (c) 2003 SKC, Inc.
@@ -36,6 +36,9 @@ static const unsigned __int64 epoch = 116444736000000000L;
 
 /*
  * timezone information is stored outside the kernel so tzp isn't used anymore.
+ *
+ * Note: this function is not for Win32 high precision timing purpose. See
+ * elapsed_time().
  */
 int
 gettimeofday(struct timeval * tp, struct timezone * tzp)