doc: add reference to uv_update_time on uv_timer_start

PR-URL: https://github.com/libuv/libuv/pull/1079
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

docs/src/timer.rst

@@ -42,6 +42,9 @@ API
     If `repeat` is non-zero, the callback fires first after `timeout`
     milliseconds and then repeatedly after `repeat` milliseconds.
 
+    .. note::
+        Does not update the event loop's concept of "now". See :c:func:`uv_update_time` for more information.
+
 .. c:function:: int uv_timer_stop(uv_timer_t* handle)
 
     Stop the timer, the callback will not be called anymore.