docs(timer): use precise delay description in Timer Handler (#4235)

docs/porting/os.rst

@@ -29,10 +29,11 @@ Here is some pseudocode to illustrate the concept:
    void lvgl_thread(void)
    {
        while(1) {
+           uint32_t time_till_next;
            mutex_lock(&lvgl_mutex);
-           lv_task_handler();
+           time_till_next = lv_task_handler();
            mutex_unlock(&lvgl_mutex);
-           thread_sleep(10); /* sleep for 10 ms */
+           thread_sleep(time_till_next); /* sleep for a while */
        }
    }
 

docs/porting/timer_handler.rst

@@ -11,16 +11,13 @@ periodically in one of the following:
 - timer interrupt periodically (lower priority than :cpp:func:`lv_tick_inc`)
 - an OS task periodically
 
-The timing is not critical but it should be about 5 milliseconds to keep
-the system responsive.
-
 Example:
 
 .. code:: c
 
    while(1) {
-     lv_timer_handler();
+     uint32_t time_till_next = lv_timer_handler();
-     my_delay_ms(5);
+     my_delay_ms(time_till_next);
    }
 
 If you want to use :cpp:func:`lv_timer_handler` in a super-loop, a helper
@@ -30,9 +27,9 @@ the porting:
 .. code:: c
 
    while(1) {
-       ...
+      ...
-       lv_timer_handler_run_in_period(5); /* run lv_timer_handler() every 5ms */
+      lv_timer_handler_run_in_period(5); /* run lv_timer_handler() every 5ms */
-       ...
+      ...
    }
 
 In an OS environment, you can use it together with the **delay** or
@@ -41,8 +38,8 @@ In an OS environment, you can use it together with the **delay** or
 .. code:: c
 
    while (1) {
-       lv_timer_handler_run_in_period(5); /* run lv_timer_handler() every 5ms */
+      uint32_t time_till_next = lv_timer_handler(); 
-       my_delay_ms(5);                    /* delay 5ms to avoid unnecessary polling */
+      os_delay_ms(time_till_next); /* delay to avoid unnecessary polling */
    }
 
 To learn more about timers visit the `Timer </overview/timer>`__