lua: clarify event.recurrent() API

It was rather confusing:
- event.after(0, ...) executed function immediatelly
- event.recurrent(0, ...) executed function immediatelly (seemingly worked)
  but stoped after the first execution, i.e. no recurrence took place.

daemon/bindings/event.c

@@ -94,7 +94,8 @@ static int event_recurrent(lua_State *L)
 {
 	/* Check parameters */
 	int n = lua_gettop(L);
-	if (n < 2 || !lua_isnumber(L, 1) || !lua_isfunction(L, 2))
+	if (n < 2 || !lua_isnumber(L, 1)  || lua_tointeger(L, 1) == 0
+		|| !lua_isfunction(L, 2))
 		lua_error_p(L, "expected 'recurrent(number interval, function)'");
 
 	return event_sched(L, 0, lua_tointeger(L, 1));

daemon/bindings/event.rst

@@ -25,7 +25,7 @@ For example, ``5 * hour`` represents five hours, or 5*60*60*100 milliseconds.
 
    :return: event id
 
-   Similar to :func:`event.after()`, periodically execute function after ``interval`` passes.
+   Execute function immediatelly and then periodically after each ``interval``.
 
    Example: