Core: fixed comment about ngx_current_msec after 81fae70d6cb8.

The value is no longer guaranteed to be based on milliseconds
elapsed since Epoch.

src/core/ngx_times.h

diff --git a/src/core/ngx_times.h b/src/core/ngx_times.h
index 94aedcd..49e0a8c 100644
--- a/src/core/ngx_times.h
+++ b/src/core/ngx_times.h
@@ -43,8 +43,8 @@
 extern volatile ngx_str_t    ngx_cached_syslog_time;
 
 /*
- * milliseconds elapsed since epoch and truncated to ngx_msec_t,
- * used in event timers
+ * milliseconds elapsed since some unspecified point in the past
+ * and truncated to ngx_msec_t, used in event timers
  */
 extern volatile ngx_msec_t  ngx_current_msec;