Doc: a timer must have a non-zero timeout value to be armed properly

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

API.md

@@ -69,7 +69,7 @@ int uev_io_set      (uev_t *w, int fd, int events);
 int uev_io_start    (uev_t *w);
 int uev_io_stop     (uev_t *w);
 
-/* Timer watcher:   schedule a relative timer, timeout and period in milliseconds */
+/* Timer watcher:   schedule a relative timer, timeout (must be non-zero) and period in milliseconds */
 int uev_timer_init  (uev_ctx_t *ctx, uev_t *w, uev_cb_t *cb, void *arg, int timeout, int period);
 int uev_timer_set   (uev_t *w, int timeout, int period); /* Change timeout or period */
 int uev_timer_start (uev_t *w);                          /* Restart a stopped timer */

src/timer.c

@@ -93,9 +93,13 @@ int uev_timer_init(uev_ctx_t *ctx, uev_t *w, uev_cb_t *cb, void *arg, int timeou
 /**
  * Reset a timer
  * @param w        Watcher to reset
- * @param timeout  Timeout in milliseconds before @param cb is called
+ * @param timeout  Timeout in milliseconds before @param cb is called, zero disarms timer
  * @param period   For periodic timers this is the period time that @param timeout is reset to
  *
+ * Note, the @param timeout value must be non-zero.  Setting it to zero
+ * will disarm the timer.  This is the underlying Linux function @func
+ * timerfd_settimer() which has this behavior.
+ *
  * @return POSIX OK(0) or non-zero with @param errno set on error.
  */
 int uev_timer_set(uev_t *w, int timeout, int period)