Update some cmoments

drivers/syslog/vlowsyslog.c

@@ -62,6 +62,10 @@
  *   equivalent to vlowsyslog() except that the pre-process priority filtering
  *   has already been performed and, hence, there is no priority argument.
  *
+ *   NOTE:  The va_list parameter is passed by reference.  That is because
+ *   the va_list is a structure in some compilers and passing of structures
+ *   in the NuttX sycalls does not work.
+ *
  ****************************************************************************/
 
 int _lowvsyslog(FAR const IPTR char *fmt, FAR va_list *ap)

drivers/syslog/vsyslog.c

@@ -60,6 +60,10 @@
  *   equivalent to vsyslog() except that the pre-process priority filtering
  *   has already been performed and, hence, there is no priority argument.
  *
+ *   NOTE:  The va_list parameter is passed by reference.  That is because
+ *   the va_list is a structure in some compilers and passing of structures
+ *   in the NuttX sycalls does not work.
+ *
  ****************************************************************************/
 
 int _vsyslog(FAR const IPTR char *fmt, FAR va_list *ap)

include/nuttx/syslog/syslog.h

@@ -214,6 +214,10 @@ int syslog_flush(void);
  *   equivalent to vsyslog() except that the pre-process priority filtering
  *   has already been performed and, hence, there is no priority argument.
  *
+ *   NOTE:  The va_list parameter is passed by reference.  That is because
+ *   the va_list is a structure in some compilers and passing of structures
+ *   in the NuttX sycalls does not work.
+ *
  ****************************************************************************/
 
 int _vsyslog(FAR const IPTR char *src, FAR va_list *ap);

libc/syslog/lib_lowsyslog.c

@@ -78,7 +78,12 @@ int lowvsyslog(int priority, FAR const IPTR char *fmt, va_list ap)
 
   if ((g_syslog_mask & LOG_MASK(priority)) != 0)
     {
-      /* Perform the _lowvsyslog system call */
+      /* Perform the _lowvsyslog system call.
+       *
+       * NOTE:  The va_list parameter is passed by reference.  That is
+       * because the va_list is a structure in some compilers and passing
+       * of structures in the NuttX sycalls does not work.
+       */
 
       ret = _lowvsyslog(fmt, &ap);
     }

libc/syslog/lib_syslog.c

@@ -67,7 +67,12 @@ int vsyslog(int priority, FAR const IPTR char *fmt, va_list ap)
 
   if ((g_syslog_mask & LOG_MASK(priority)) != 0)
     {
-      /* Yes.. lPerform the _vsyslog system cal */
+      /* Yes.. lPerform the _vsyslog system call.
+       *
+       * NOTE:  The va_list parameter is passed by reference.  That is
+       * because the va_list is a structure in some compilers and passing
+       * of structures in the NuttX sycalls does not work.
+       */
 
       ret = _vsyslog(fmt, &ap);
     }