doc: document uv_timer_start() on an active timer

Fixes: https://github.com/libuv/libuv/issues/1401
PR-URL: https://github.com/libuv/libuv/pull/1538
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

docs/src/timer.rst

@@ -45,6 +45,8 @@ API
     .. note::
         Does not update the event loop's concept of "now". See :c:func:`uv_update_time` for more information.
 
+        If the timer is already active, it is simply updated.
+
 .. c:function:: int uv_timer_stop(uv_timer_t* handle)
 
     Stop the timer, the callback will not be called anymore.